I haven't played a whip user before, but I did come up with the following build.
Whip Master (Fighter)
Key concepts: Whip using Fighter
Race: Human
Class: Fighter
Traits: Adopted(Tiefling), Chain Master,
Feat chain: Exotic WP[Whip](1st), Dirty Fighting(1st), Improved Trip(3rd), Combat Reflexes(5th)
Bonus Feats: Weapon Focus[Whip](1st), Whip Mastery(2nd), Toppling Pileup (4th), Greater Trip(6th)
Suggested Feats: Improved Whip Mastery, serpent lash, vicious stomp, felling smash
The way this character works is he uses his whip to trip one opponent. If the combat maneuver succeeds two things happen.
1. the person he just tripped provokes an AoO which he can use to attack or disarm them.
2. he can immediately try to trip someone adjacent to the person he just tripped. If that person falls down they also provoke an AoO which he can use to attack or disarm.
Then, if someone tries to stand that also provokes an AoO giving him yet another attack.
Now, if you wanted you could swap out Toppling Pileup and Combat Reflexes for power attack and felling smash.
This changes things where instead he makes one attack against an enemy. If the attack hits he can trip them for free and if he trips them they provoke an AoO.
The first option spreads the damage out more, whereas the 2nd option lets him focus fire on a single enemy. I didn't see any archetypes that support a whip or trip build but it's possible there is an archetype that give you some of these feats for free, which would give you additional feats to work with.
edit: i realized the build needs improved whip mastery to work properly. an easy way to f8x th8s is by giving him a whip enchanted with training and one of the feats i listed. perhaps exotic weapon proff whip. so its still userul as loot.
Humans have an alternative racial feature called military tradition which gives EWP in TWO weapons. This replaces the human bonus feat. So you could grab the Whip and Scorpion Whip.
Be a Oppunist Fighter/Mutagen Warrior with the Accelerated Drinker trait. Whip stuff, throw bombs at things. If he survives to see level 7, take the Wings Discovery.
Or...
Go Lore Warden Fighter and use the bonus to maneuvers to make him a maneuver master following the build LordKailas posted.
Or...
Look at the Vengeful Hunter archetype, as it gives whip proficiency.
Human
Heart of the Fey
Military Tradition
Drawback
Cruelty or Warded Against Nature
Traits
Accelerated Drinker
Adopted (Tiefling) Chain Master
Armor Master for mithral breastplate?
Pragmatic Activator? Something for Will saves, maybe.
Opportunist Fighter/Mutagen Warrior
1(class bonus). Imp. Dirty Trick
1H. EWP Whip
1H. EWP Scorpion Whip
1. Weapon Finesse
2B. Dirty Fighting
3. Mutagen! Su.
3. Imp. Trip
4. Bombs! Ex.
5. Combat Reflexes
6. Great Trip
Use a +1 Agile Scorpion Whip for dex to damage.
Use a DEX Mutagen for better AC, better Fly checks if you have wings, better accuracy with your bombs and whip, and more damage with your whip.
If you go to level 7, you can take the Wings Discovery, and fly above dropping bombs and tagging them with your whip.
You get Know: Nature and Perception from Heart of the Fey (and low light vision). As well as Bluff, Sense Motive, Sleight of Hand, and Stealth from Opportunist Fighter.
Opportunist Fighter also gives you Alchemical Onslaught Ex. @5:
Adding +1 to attacks with alchemical weapons and INT to damage with splash weapons. You can draw an alchemical weapon as a free action once per round.
Retrain Imp. Dirty Trick for TWF and equip him with some Enlarge Person potions... he is good, clean fun.
How does the Mutation Warrior get bombs?
Opportunist Fighter Cunning Edge is how he gets Bombs.
At level 8, he goes into Vivisectionist Alchemist for Sneak Attack with his whip and bombs... at level 9 he gets the Deathwatch Extract (which will go awesome with the Cruelty Drawback).
If you outfit him with an Elven Cloak and spiked mithral breastplate (assuming you take the Armor Master regional trait) he is supervillain material at level 9. He will have Mutagen, Agile Scorpion Whip, Bombs, Great Trip, Wings, Sneak Attack, Bleeding Attack Rogue Talent, and Extracts...
For his tripping attempts he gets:
Great Trip is a +4, +1 weapon enchantment, +2 trait, +2 trip weapon, +1 size, +20 from a True Strike Extract, +BAB, +DEX... all at the reach of a size large whip. This should be sufficient to trip most PC's.
Max skill ranks in:
Perception
Know: Nature
Stealth
Craft: Alchemy
UMD and Fly when you get them
Through Vivisectionist, he can use Know: Nature in place of Heal.
At level 7 he can take Hover or Fly-By Attack.
At level 9 he can take Accomplished Sneak Attacker.
Mithral Breastplate is +6 AC, +5 DEX bonus, -0 ACP (w/ Armor Master trait) 15# (20# w/ cold iron spikes, 17.5 w/mithral spikes).
You could skip the Elven Cloak and just make the mithral breastplate +1 with the Shadow enchantment on it. Whatever works. I just liked the thought of him being able to hide his wings and whip under the cloak.
With the racial +2 floating stat increase, and two stat increases due to levels, he should have a Dexterity of 20 without belts pretty easily. That maxes out the DEX bonus on his armor for now.
Keep his Intelligence as his priority mental stat for skills and the Alchemist stuffs. It also determines how many bombs you can make, and the extra damage you do with them.
Keep his inventory loaded with Enlarge Person potions, and use the Accelerated Drinker trait to his advantage. The increased reach and damage will be welcome.
If they get the jump on him, or charge inside his reach, make AoO with the armor spikes and Combat Reflexes. The additional Strength from being Large will help the damage of the armor spikes, since they probably won't be Agile, yet (if ever).
Withdraw, flying straight up at half speed (so 30'). Whip them from this distance with your size large whip. Drop bombs on them.
Use TWF for whip/whip/bomb every full attack.
If he ever gets to use his Sneak Attack, the Bleeding Attack Rogue Talent adds a little more cruelty to his character.
If you continue past level 9 with him, pursue the whip mastery feats to threaten AoO at full range with the whip.
And look into the Spontaneous Healing, Preserved Organs, and Mummification Discoveries that will keep him alive.
After he can threaten AoO at full range with the whip, his full attack sequence goes:
Trip w/AoO, Trip/AoO, Trip w/AoO, bomb...
And that is good, clean fun from 30' with a large whip. He uses Trip, because unarmed and natural attacks still often originate from things that can be tripped. And you can trip everything with a little bit of True Strike Extract...
His Fortitude save is baller, his Reflex gets a little jumpstart, and his Will save doesn't really miss a beat, overall. Still needs Iron Will or the proper Ioune Stones to help out, like any class will slow Will saves.
Upgrade the whip to +2 Speed. Or the armor to help out with your AC. Depending upon if you need damage or defenses, obviously.
At level 19 you can take Crippling Strike Rogue Talent instead of a Discovery, you should.
Since Weapon Focus is a prerequisite for the whip mastery feats, try to take Weapon Versatility to overcome Damage Reduction.
Being able to fly, and threatening 30' when size large, opens up a ton of flanking opportunities for Sneak Attack damage... just silently hovering in the shadows, lurking, waiting to strike.
You petty slaves never know when the warden is watching...
IF this NPC is a villain, which I assume since he is a slave master...
Introduce him at level 7, with his wings and whip under the Elven Cloak. He takes his Mutagen in the surprise round. His Enlarge Person potion first round of battle... reverse order as necessary.
He keeps his wings hidden in the first encounter until he is ganged up on within his whip's reach, or brought below half health. He uses his wings to withdraw, or at least move, and drink a potion of Invisibility to escape. Always include minions to leave behind.
The next time he is introduced, he is level 9, and has sneak attack with Bleeding Attack, makes a point to use his bombs, and has Extracts like True Strike. He focuses much more on Stealth than the last time he was seen. Make sure his minions always flank adjacent opponents, so his armor spikes get sneak attack damage.
After he has the whip mastery feats and threatens AoO at full range with the whip, his minions always are positioned to attack the people he trips, and collapse into flanking positions if everyone rushes you.
If he is a friendly NPC, the change from Fighter to Alchemist shows he isn't a slave master monster crap person, maybe he has changed from brute to brilliance.
@ level 7: BAB +7/+2, Base Saves +7,+2,+2
Bombs 2D6
@ level 9: BAB +8/+3, Base Saves +10,+5,+2
Bombs 2D6, Sneak Attack 2D6