Madness Tarrasquekiller: Slaying the Tarrasque as a 1st level commoner.

[Inevitability]

a commoner would almost instantly fail his wisdom check vs the allip and go about as crazy. The allip does not really have a motivation to kill the tarrasque. At lower wisdom i bet the tarrasque would be raging and destroying, most things. I believe you have to cast your spell as well as actually land a killing blow (good luck commoner doing that to something with crazy high AC) Also not really seeing anything allowing you to get a 9th level spell slot or the spell miracle.

If you are afraid of going Allip-Crazy, buy some wax and put it in your ears. This is explicitly stated to prevent Sonic effects from working.

Indeed, the Allip has no motivation. That's why you are Commanding it to do your bidding. Allips are CE and quite insane, so I would be surprised if it doesn't follow a trail of fleshy, drainable bodies (your chickens).

No, you do not have to land the killing blow. You just need to cast Miracle or Wish at the unconscious Tarrasque's body, and then it dies.

The spell miracle is gained by the Arcane Disciple feat, which lets you add Miracle to your spell list. (yes you have one, Magical Training gives you a spell list)
The spell slot is gained by the Pact, which gives you a spell slot of any level.

This character is a caster with a spell slot of the intended level, an ability score high enough to cast the spell and the spell on his spell list. What else would he need to cast the spell? I doubt Kurtulmak would be very opposed to slaying the Tarrasque.

Also no DM would allow half these things to work. May work in a purely technical sense but easily screwed up and in practice not really viable (for instance asking for a knowledge check the DM could say a commoner would not know how to make a demonic pact ect)

Of course a DM wouldn't allow this. This is TO. The point is that this works by RAW, and if a DM goes restricting parts of it, then that isn't this build's problem.