What he should've done:
1) Ask girl to believe that his power is "The ability to create anything"
2) Create a copy of himself with his memories
3) Have his copy believe that he has the power to create and give people abilities
4) Give his copy the ability to see and change other people's abilities
5) Have his copy change his ability from "Sophist" to "Omnipotent", so that he basically becomes a god
6) Absorb his copy into himself so that their consciousness's and memories merge
7) Destroy all other abilities (Cus of the copy girl)
8) Win everything
The reason I included the step to "Make a copy of himself" instead of just telling the girl, is because the girl would probably be really suspicious of him if he asked her to change his ability to ""Omnipotent". And the reason I had him create a clone of himself instead of just immediately changing his ability, is because if he changed his ability immediately, it could potentially backfire with him only getting a partial ability, or losing his ability completely. And the reason why he should absorb his clone's consciousness and memories, is because his clone would be very dangerous and would also not want to die, so by merging together, it quells any sort of backlash as the clone still kinda exists.
I thought he was supposed to be some kind of logic master, why hasn't he thought of this? XD