I just lost an entire detailed post describing the fight hit for hit. :( This will be more succinct.
I ran a 14th level summoner 'mage' with a 'pit fiend' for an eidolon, against a 10th level half elf fighter (two handed), a 10th level human fighter (sword n board), a 10th level phanaton druid (big cat companion) and a human rogue 1/conjurer 9. They had a 7th level NPC Olman bard with them.
I have a houserule that at 10th level in a 6 or 9 level spellcaster, your 1st level spells become at will. It did not come into play, but for those who critique the character sheet, that's why it says at will.
The summoner brought out a dretch during the pre-fight, and an osyluth once the real fight began. I ignored the 'only 1 at a time' rule, because the dretch was more of a flavor piece than an actual combatant. The nasty little Aporcus Beedle, formerly of Freeport, from Sasserine in my campaign.
I made some tactical errors with the mage, not flying was a big one. Without a decent AC (something all NPCs struggle to maintain without gobs of inappropriate treasure), and no way to negate damage, he took several full attacks from the fighters (causing him to drop to 1 hp and the eidonlon to begin taking damage) before the animal growth'd sabertooth tiger companion of the druid finished him and the eidolon off with a charging pounce-rake-critical that dealt over 140 damage.
The eidolon managed to hit the low AC fighter, but missed the sword and boarder badly. The osyluth missed nearly all of its attacks. The summoner used mass bear's endurance and hold monster, both of which were effective.
I played up the eidolon's evasion as the missing SR, which I forced the players to roll caster level checks for. Luckily, they failed the same checks that were successful saves for the eidolon, so the illusion that they were fighting a mage and a pit fiend with some sort of life link was maintained throughout nearly the entire fight. They had their suspicions at the end, which I deflected by not actually answering but simply asking them more questions. (Nor am I too concerned if they find the truth here.)
Not using fly and haste were two big errors on my part. If I had, the fight would have lasted a bit longer and possibly been more dangerous. As it was, however, a single 14th level NPC did manage to hang with a party that kills pairs of advanced T-Rexes with ease.
I find the summoner to be an intriguing concept. The eidolon was very cool to build. Had I paired him with another NPC, or placed him in an actual group with real minions in addition to his summons, he could have caused quite a battle royale.
Arakk Beedle, seeker of Fiend's Embrace
XP 19,200
Male human summoner 14
CE Medium Humanoid (Human)
Init +3; Senses Perception +6
Languages Abyssal, Common, Dwarven, Elven, Infernal
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AC 24, touch 15, flat-footed 21 (+6 armor, +2 deflection, +3 Dex, +1 natural, +4 shield), combat 27
hp 163 (14d8 + 70)
Fort +12, Ref +10, Will +11
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Speed 30 ft.
Melee Rapier +14/+8 (1d6 + 3 /18-20) or
Rapier vs humans +16/+11 (3d6 + 5 /18-20)
Space 5 ft.; Reach 5 ft.
Base Atk +10; Combat +12
Special Actions master's call, summon monster VII
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Summoner Spells known (Caster level 14th)
5th (4/day)- greater dispel magic, greater teleport, spell turning
4th (5/day)- baleful polymorph (DC 21), mass bear's endurance, dismissal, hold monster (DC 21)
3rd (6/day)- charm monster (DC 20), dispel magic, fly, magic circle, wall of ice
2nd (7/day)- alter self, blur, haste, invisibility, resist energy, see invisibility
1st (At will) – feather fall, grease, mount, shield, unseen servant, ventriloquism
0 (At will) – arcane mark, detect magic, light, mage hand, mending, read magic
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Abilities Str 15, Dex 17, Con 20, Int 18, Wis 9, Cha 24
SQ bond senses, life bond, life link
Feats Combat Casting, Quick Draw, Weapon Focus (rapier)
Skills Appraise +20, Bluff +23, Fly +19, Knowledge (arcana) +20, Knowledge (local) +20, Knowledge (the planes) +20, Perception +6, Spellcraft +20
Possessions +2 chain shirt, +1 human bane rapier, cloak of resistance +3, amulet of natural armor +1, ring of protection +2, a pouch containing a deck of cards, spellbook containing (1st - alarm, endure elements, floating disk, hold portal, Isle mount, silent image, unseen servant; 2nd - acid arrow, continual flame, darkvision, resist energy; 3rd - greater magic weapon, major image, phantom steed, water breathing)
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Master's Call 3/day as a standard action, Arrak can summon his eidolon to his side as per dimension door. He can also use this ability to change places with his eidolon.
Summon 10/day, Arrak can duplicate the effects of summon monster VII as a standard action. These summons remain for 14 minutes.
Life Bond Damage that would kill Arrak is instead transferred to his eidolon.
Life Link Arrak may sacrifice his own hit points to prevent damage to his eidolon that would send it back to its home plane.
The mighty Infernus, Pit Fiend Spirit (formerly possessed Fiend's Embrace...as it was carved from his flesh.)
CE Large Outisder
Init +3; Senses Darkvision, Perception +15
Languages Abyssal, Common, Infernal
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AC 26, touch 12, flat-footed 23 (+3 Dex, +14 natural, -1 size), combat 37
hp 138 (12d10 + 72); DR 10/good
Resist fire 15
Fort +13, Ref +7, Will +8; evasion
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Speed 30 ft., fly 30 ft. (average); Fly +18
Melee bite +22 (1d8 + 11 and poison) and
2 claws +20 (1d6 + 5) and
tail +20 (1d8 + 5 plus grab) and
2 wings +20 (1d6 + 5)
Space 5 ft.; Reach 5 ft.
Base Atk +12; Combat +24 (+28 grapple)
Atk Options power attack (-4, +8), improved vital strike
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Abilities Str 33, Dex 16, Con 21, Int 9, Wis 11, Cha 12
SQ devotion, link, share spells
Feats Fly-by Attack, Improved Fly-by Attack, Improved Vital Strike, Multiattack*, Power Attack, Toughness, Vital Strike
Skills Fly +18, Intimidate +16, Knowledge (the planes) +11, Perception, +15, Sense Motive +15
Possessions none
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Poison Injury, DC 21, 1d4 Con (4)
Building the Eidolon....
20 pts
DR 10/good (5 pt)
Large (3)
Bite (1)
Flight (2)
Wings (1)
Tail (1)
Poison (4)
Grab (2)
Resist fire 15 (1)
Not used (but good for more advanced 'pit fiend eidolons')
SR 25 (4)
Imp damage (1)
Magic attacks (1)
Immune fire (2)
Resist cold 15 (1)
Resist acid 15 (1)
Constrict (2)