How many members in the party?
I'm not going to go building a character at the moment, but something worth while to keep in mind for boss encounters is action economy. Even if you have a really tough big baddie, but it's alone, your party is (likely) going to beat it down real quickly because of their greater amount of actions.
I recommend reading this guide, it has some really good points on how to make a boss encounter challenging, rewarding, and memorable.
Here's Spicy Shoes normally. He should be somewhere above a ledge throwing poisons to the party, perhaps chugging Reduce Person to dodge ranged attacks:
Male Ratfolk alchemist 4 Archetypes Vivisectionist,
None Small humanoid (ratfolk)
Init +4, Senses Perception +6
=================================================
DEFENSE
=================================================
AC 20 (22 when cornered), touch 16, flat-footed 16 (+4 armor, +4 Dex, +1 size )
hp 27 ((4d8)+8)
Fort +6, Ref +9, Will +1, +2 vs. poison
=================================================
OFFENSE
=================================================
Speed 20 ft.
Ranged alchemist's fire (flask) +7 (1d6+3)
Ranged ghast retch (flask) +7 (/none/x0)
Ranged thunderstone +7 (/none/x0)
Ranged itching powder +7 (/none/x0)
Ranged tangleburn bag +7 (1d6+3/none/x0)
Ranged sneezing powder +7 (/none/x0)
Special Attacks Feral Mutagen, Sneak Attack 2d6, Throw Anything,
Prepared Spells
Alchemist (CL 4th; concentration +7)
1st-enlarge person(DC 14), expeditious retreat(DC ), reduce person(DC 14)
=================================================
STATISTICS
=================================================
Str 14 (16 when attacking), Dex 18, Con 12, Int 16, Wis 8, Cha 8,
Base Atk +3; CMB +4; CMD 19
Feats Brew Potion, Desperate Battler, Extra Bombs, Small But Deadly, Throw Anything
Skills Climb +4, Craft (Alchemy) +12, Escape Artist +6, Knowledge (Nature) +10, Perception +6, Spellcraft +10, Survival +6,
Traits Accelerated Drinker, Cheat Death (Besmara),
Languages Common, Dwarven, Goblin, Undercommon
SQ alchemy, brew potion, cornered fury, cruel anatomist, darkvision, mutagen, poison resistance, poison use, sneak attack, spontaneous healing, swift alchemy, tinker, torturer's eye, unnatural,
Combat Gear alchemist's fire (flask) (2), ghast retch (flask), antiplague (vial), antitoxin (vial) (2), potion of barkskin +2, potion of bull's strength, potion of cure light wounds (2), potion of cure moderate wounds, potion of magic fang (2), pox burster, thunderstone, itching powder, tangleburn bag (2), sneezing powder,
Other Gear filter hood (small), ring of protection +1, cloak of resistance +1, chain shirt (small), belt of tumbling, formula book, 2250.0 gp
=================================================
SPECIAL ABILITIES
=================================================
Accelerated Drinker You know how to drink a potion efficiently, such as by not using your hands, tossing it in the air and catching it in your mouth, or opening it with your teeth. You may drink a potion as a move action instead of a standard as long as you start your turn with the potion in your hand.
Alchemy (Su) When using Craft (Alchemy) to create an alchemical item, you gains a +4 competence bonus on the Craft (alchemy) check. In addition, you can use Craft (Alchemy) to identify potions as if using Detect Magic. He must hold the potion for 1 round to make such a check.
Brew Potion (Ex) You receive Brew Potion as a bonus feat. You can brew potions of any formulae you know (up to 3rd level), using your alchemist level as caster level. The spell must be one that can be made into a potion. You do not need to meet the prerequisites for this feat. Alchemists receive the Extra Bombs feat at 1st level instead of Brew Potion.
Cheat Death (Besmara) Even Besmara's most faithful worshipers usually call upon her only in times of greatest need. Sometimes Besmara intervenes on behalf of her faithful; other times, she turns her face away as they sink beneath the waves. Once per day, when you would be reduced to 0 or fewer hit points as a result of a failed ability check, skill check, or saving throw on your part, you may invoke this ability in order to reroll the failed check. You must take the result of the second roll, even if it is worse than the original.
Cornered Fury Ratfolk can fight viciously when cut off from friends and allies. Whenever a ratfolk with this racial trait is reduced to half or fewer of his hit points, and has no conscious ally within 30 feet, he gains a +2 racial bonus on melee attack rolls and to Armor Class. This racial trait replaces swarming.
Cruel Anatomist (Ex) You may use your Knowledge (Nature) skill bonus in place of your Heal skill bonus.
Darkvision Ratfolk can see in the dark up to 60 feet
Feral Mutagen Whenever the alchemist imbibes a mutagen, he gains two claw attacks and a bite attack. These are primary attacks and are made using the alchemist's full base attack bonus. The claw attacks deal 1d6 points of damage (1d4 if the alchemist is Small) and the bite attack deals 1d8 points of damage (1d6 if the alchemist is Small). While the mutagen is in effect, the alchemist gains a +2 competence bonus on Intimidate skill checks.
Mutagen (Su) You know how to create a mutagen that you can imbibe in order to heighten your physical prowess at the cost of your personality. It takes 1 hour to brew a dose of mutagen, and once brewed, it remains potent until used. You can only maintain one dose of mutagen at a time - if you brews a second dose, any existing mutagen becomes inert. A mutagen that is not in your possession becomes inert until an alchemist picks it up again. When you brew a mutagen, you select one physical ability score - either Strength, Dexterity, or Constitution. It's a standard action to drink a mutagen. Upon being imbibed, the mutagen causes you to grow bulkier and more bestial, granting you a +2 natural armor bonus and a +4 alchemical bonus to the selected ability score for 40 minutes. In addition, while the mutagen is in effect, you take a -2 penalty to one of your mental ability scores. If the mutagen enhances your Strength, it applies a penalty to your Intelligence. If it enhances your Dexterity, it applies a penalty to your Wisdom. If it enhances your Constitution, it applies a penalty to your Charisma. A non-alchemist who drinks a mutagen must make a DC 15 Fortitude save or become nauseated for 1 hour - a non-alchemist can never gain the benefit of a mutagen, but an alchemist can gain the effects of another alchemist's mutagen if he drinks it. (Although if the other alchemist creates a different mutagen, the effects of the "stolen" mutagen immediately cease.) The effects of a mutagen do not stack. Whenever an alchemist drinks a mutagen, the effects of any previous mutagen immediately end.
Poison Resistance (Ex) You gain a +2 bonus on all saving throws against poison.
Poison Use (Ex) You are trained in the use of poison and cannot accidentally poison yourself when applying poison to a weapon.
Sneak Attack (Ex) If you can catch an opponent when he is unable to defend himself effectively from your attack, you can strike a vital spot for extra damage. [0d6]
Sneak Attack (Ex)
Spontaneous Healing (Ex) You gain the ability to heal from wounds rapidly. As a free action once per round, you can heal 5 hit points as if you had the fast healing ability. You can heal 10 hit points per day in this manner. If you fall unconscious because of hit point damage and you still have healing available from this ability, the ability activates automatically each round until you are conscious again or the ability is depleted for the day.
Swift Alchemy (Ex) You can create alchemical items with astounding speed. It takes you half the normal amount of time to create alchemical items, and you can apply poison to a weapon as a move action.
Throw Anything (Ex) You gain the Throw Anything feat as a bonus feat. You add your Intelligence modifier to damage done with splash weapons, including the splash damage if any. This bonus damage is already included in the bomb class feature.
Tinker (Ex) Ratfolk gain a +2 racial bonus on Craft (Alchemy), Perception, and Use Magic Device checks.
Torturer's Eye (Ex) You add Deathwatch to your formula book as a 1st-level extract.
Unnatural Some ratfolk unnerve normal animals, and train to defend themselves against the inevitable attacks from such creatures. These ratfolk take a -4 penalty on all Charisma-based skill checks to affect creatures of the animal type, and receive a +2 dodge bonus to AC against animals. An animal's starting attitude toward ratfolk is one step worse than normal. This racial trait replaces rodent empathy.
As the party comes close to him, he'll drink the Enlarge and Expeditious Retreat potions, Magic Fang and Barskin, and turn on his mutagen to boot. His great saves vs. poisons (remember he has the mask on and several good +saves for poison consumables) allow him to just drop poisons right in front of him without much fear of poisoning himself. This is how he'll look:
Male Ratfolk alchemist 4 Archetypes Vivisectionist,
Large humanoid (ratfolk)
Init +3, Senses Perception +6
=================================================
DEFENSE
=================================================
AC 20 (+2 AC when cornered), touch 14, flat-footed 16 (+4 armor, +3 Dex, +2 alchemical)
hp 31 ((4d8)+12)
Fort +7, Ref +8, Will +1, +2 vs. poison
=================================================
OFFENSE
=================================================
Speed 50 ft.
Melee +10 claw/claw/bite (1d8+10/1d8+10/1d10+10) --note you get +2 from Corned Fury and +1 from Desperate Battler, taking -5 for multiattacking too--
Ranged alchemist's fire (flask) +6 (1d8+3)
Ranged ghast retch (flask) +6 (/none/x0)
Ranged thunderstone +6 (/none/x0)
Ranged itching powder +6 (/none/x0)
Ranged tangleburn bag +6 (1d8+3/none/x0)
Ranged sneezing powder +6 (/none/x0)
Special Attacks Feral Mutagen, Sneak Attack 0d6, Throw Anything,
Prepared Spells
Alchemist (CL 4th; concentration +7)
1st-enlarge person(DC 14), expeditious retreat(DC ), reduce person(DC 14)
=================================================
STATISTICS
=================================================
Str 24 (26 when attacking), Dex 16, Con 14, Int 14, Wis 8, Cha 8,
Base Atk +3; CMB +8; CMD 22
Feats Brew Potion, Desperate Battler, Extra Bombs, Small but Deadly, Throw Anything
Skills Climb +7, Craft (Alchemy) +12, Escape Artist +5, Knowledge (Nature) +10, Perception +6, Spellcraft +10, Survival +6,
Traits Accelerated Drinker, Cheat Death (Besmara),
Languages Common, Dwarven, Goblin, Undercommon
SQ alchemy, brew potion, cornered fury, cruel anatomist, darkvision, mutagen, permanency spell / enlarge person (09), permanency spell / magic fang (09), poison resistance, poison use, sneak attack, spontaneous healing, swift alchemy, tinker, torturer's eye, unnatural,
Combat Gear alchemist's fire (flask) (2), ghast retch (flask), antiplague (vial), antitoxin (vial) (2), potion of barkskin +2, potion of bull's strength, potion of cure light wounds (2), potion of cure moderate wounds, potion of magic fang (2), pox burster, thunderstone, itching powder, tangleburn bag (2), sneezing powder,
Other Gear filter hood (small), ring of protection +1, cloak of resistance +1, chain shirt (small), belt of tumbling, formula book, 2250.0 gp
=================================================
SPECIAL ABILITIES
=================================================
Accelerated Drinker You know how to drink a potion efficiently, such as by not using your hands, tossing it in the air and catching it in your mouth, or opening it with your teeth. You may drink a potion as a move action instead of a standard as long as you start your turn with the potion in your hand.
Alchemy (Su) When using Craft (Alchemy) to create an alchemical item, you gains a +4 competence bonus on the Craft (alchemy) check. In addition, you can use Craft (Alchemy) to identify potions as if using Detect Magic. He must hold the potion for 1 round to make such a check.
Brew Potion (Ex) You receive Brew Potion as a bonus feat. You can brew potions of any formulae you know (up to 3rd level), using your alchemist level as caster level. The spell must be one that can be made into a potion. You do not need to meet the prerequisites for this feat. Alchemists receive the Extra Bombs feat at 1st level instead of Brew Potion.
Cheat Death (Besmara) Even Besmara's most faithful worshipers usually call upon her only in times of greatest need. Sometimes Besmara intervenes on behalf of her faithful; other times, she turns her face away as they sink beneath the waves. Once per day, when you would be reduced to 0 or fewer hit points as a result of a failed ability check, skill check, or saving throw on your part, you may invoke this ability in order to reroll the failed check. You must take the result of the second roll, even if it is worse than the original.
Cornered Fury Ratfolk can fight viciously when cut off from friends and allies. Whenever a ratfolk with this racial trait is reduced to half or fewer of his hit points, and has no conscious ally within 30 feet, he gains a +2 racial bonus on melee attack rolls and to Armor Class. This racial trait replaces swarming.
Cruel Anatomist (Ex) You may use your Knowledge (Nature) skill bonus in place of your Heal skill bonus.
Darkvision Ratfolk can see in the dark up to 60 feet
Feral Mutagen Whenever the alchemist imbibes a mutagen, he gains two claw attacks and a bite attack. These are primary attacks and are made using the alchemist's full base attack bonus. The claw attacks deal 1d6 points of damage (1d4 if the alchemist is Small) and the bite attack deals 1d8 points of damage (1d6 if the alchemist is Small). While the mutagen is in effect, the alchemist gains a +2 competence bonus on Intimidate skill checks.
Mutagen (Su) You know how to create a mutagen that you can imbibe in order to heighten your physical prowess at the cost of your personality. It takes 1 hour to brew a dose of mutagen, and once brewed, it remains potent until used. You can only maintain one dose of mutagen at a time - if you brews a second dose, any existing mutagen becomes inert. A mutagen that is not in your possession becomes inert until an alchemist picks it up again. When you brew a mutagen, you select one physical ability score - either Strength, Dexterity, or Constitution. It's a standard action to drink a mutagen. Upon being imbibed, the mutagen causes you to grow bulkier and more bestial, granting you a +2 natural armor bonus and a +4 alchemical bonus to the selected ability score for 40 minutes. In addition, while the mutagen is in effect, you take a -2 penalty to one of your mental ability scores. If the mutagen enhances your Strength, it applies a penalty to your Intelligence. If it enhances your Dexterity, it applies a penalty to your Wisdom. If it enhances your Constitution, it applies a penalty to your Charisma. A non-alchemist who drinks a mutagen must make a DC 15 Fortitude save or become nauseated for 1 hour - a non-alchemist can never gain the benefit of a mutagen, but an alchemist can gain the effects of another alchemist's mutagen if he drinks it. (Although if the other alchemist creates a different mutagen, the effects of the "stolen" mutagen immediately cease.) The effects of a mutagen do not stack. Whenever an alchemist drinks a mutagen, the effects of any previous mutagen immediately end.
Poison Resistance (Ex) You gain a +2 bonus on all saving throws against poison.
Poison Use (Ex) You are trained in the use of poison and cannot accidentally poison yourself when applying poison to a weapon.
Sneak Attack (Ex) If you can catch an opponent when he is unable to defend himself effectively from your attack, you can strike a vital spot for extra damage. [2d6]
Spontaneous Healing (Ex) You gain the ability to heal from wounds rapidly. As a free action once per round, you can heal 5 hit points as if you had the fast healing ability. You can heal 10 hit points per day in this manner. If you fall unconscious because of hit point damage and you still have healing available from this ability, the ability activates automatically each round until you are conscious again or the ability is depleted for the day.
Swift Alchemy (Ex) You can create alchemical items with astounding speed. It takes you half the normal amount of time to create alchemical items, and you can apply poison to a weapon as a move action.
Throw Anything (Ex) You gain the Throw Anything feat as a bonus feat. You add your Intelligence modifier to damage done with splash weapons, including the splash damage if any. This bonus damage is already included in the bomb class feature.
Tinker (Ex) Ratfolk gain a +2 racial bonus on Craft (Alchemy), Perception, and Use Magic Device checks.
Torturer's Eye (Ex) You add Deathwatch to your formula book as a 1st-level extract.
Unnatural Some ratfolk unnerve normal animals, and train to defend themselves against the inevitable attacks from such creatures. These ratfolk take a -4 penalty on all Charisma-based skill checks to affect creatures of the animal type, and receive a +2 dodge bonus to AC against animals. An animal's starting attitude toward ratfolk is one step worse than normal. This racial trait replaces rodent empathy.
This is a CR 3 encounter, what most consider hard. If your team is optimized, they can take him on, but he'll be one tough cookie. If you are playing with a mostly fun group with less than optimized characters, he'll tear you a new one.
thats a lot of actions to chug that many potions if the party is closing on him.
and he's really fricking buff if he gets to do it, so that's where mobility gains importance.
I should clarify some tactics:
1. Rat starts out in high ground, tossing a volley of potions at the party.
2. Tanglefoot to slow fast ones, Thunderstone to deafen casters, ghast retch, itching powder and sneezing powder get used either on stationary ranged characters or right at the foot of the Rat, since it has very high poison saves.
3. Remember Cornered Fury only triggers when its at half health. I added the stats for it on the second block but you probably have to remove them.
4. It has some healing potions, it probably wants to use them if the party shoots at him from afar and doesn't make a huge effort to come close... this will make him outlast a volley of ranged spells. Remember that if he heals while low, he may not benefit from Cornered Fury anymore
5. A fun variant would be adding two additional CR 1 mobs and dropping the Rat to level 3. If someone makes a knowledge check on the Rat, they'll find out about Cornered Fury so they won't kill the Rat's teammates first.