How many 6th level characters does it take...

[Jack_Simth]

Ingredients:
Conjurer 6
Scroll of Gate (DC 18 CL Check)
Take 10 CL Check Feat (Can't remember what it's name is)
Agony (+2 CL, one-use)
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Gate in a 16HD mob to kill the 12HD one. Fly to avoid dying.

EDIT:
Cleric wouldn't work since it can't get Arcane Mastery

Note:
A Scroll of Gate has a caster level requirement of 17 - expressly out of the question for this exercise.

Not so, although that is a tricky point. Firstly there's no hard and fast rules for Auras that are SUs, but assuming they're treated as spells you select an intersection, not a square, as it's starting point. Since I'm 5 feet below the ground, and I take up a 5-foot space, and I'm the source of the aura, I can pick one of the four corners of the cube I occupy which are on the surface, and have line of effect from my aura to the Barbed Devil. I don't have LoE from myself to him, but this doesn't matter, observe:

So it doesn't say "centered on you" or similar, huh?

Besides which I don't have to catch him in the aura, I can hit him with my positive energy ray as a standard action and dive underground as a move action with the same results. It's touch AC to my ray is only 16, a fairly easy target for a 3rd level combat character.

Also, you'll need a lot of hits before he explodes:

Major positive-dominant planes go even further. A creature on a major positive-dominant plane must make a DC 15 Fortitude save to avoid being blinded for 10 rounds by the brilliance of the surroundings. Simply being on the plane grants fast healing 5 as an extraordinary ability. In addition, those at full hit points gain 5 additional temporary hit points per round. These temporary hit points fade 1d20 rounds after the creature leaves the major positive- dominant plane. However, a creature must make a DC 20 Fortitude save each round that its temporary hit points exceed its normal hit point total. Failing the saving throw results in the creature exploding in a riot of energy, killing it.

(Emphasis added)
In order to actually kill it this way, you need to bring it's temporary HP above it's normal HP - which means you need to apply 126 temporary HP to the critter before it risks going boom ... assuming it doesn't use it's knoweledge skill on The Planes to know it needs to rip itself apart every now and again.

Also, if you're stepping up, you're only covered every other time the beast's turn comes around, as you can only move once in a turn.

Me thinks that the time it takes to buy items/find a cohort if you took Leadership/summon your animal companion/call a familiar/gain the XP to get to level six are all things that take longer then the one minute buffing period.

But that's okay because they don't take place during the buffing period, just like finding and Rebuking an Allip (and having him massacre a village) also doesn't take place in that minute.

True enough, but fortunately the edit removed such options.

Also, note that the Devil can never actually hit in melee an incorporeal creature, so if you can soak his 1/day abilities and have fire resist up he can't do anything.

At caster level 6, Energy Resistance(Fire) only gets you Fire Resistance 10 ... and the Scorching Ray can theoretically hit for 48 (critical on max damage) although it'll generally average 14 per ray. A caster level 7 scroll, potion, or wand will usually do it... but if you're incorporeal, you've got issues using items.