You know, beating GOD MODE gives us same rewards all the time regardless how fast we kill him and how much is our hp left when we kill him.
So, maybe different rewards?
Example:
Beating GOD MODE in 12 turn or faster gives a random legendary item, Beating GOD MODE with 300+ HP left triples the sm credits and xp obtained, etc.
To avoid cheating, make sure the developer uses some cheating detector that when the cheat is detected, GOD MODE says "YOU WANT TO CHEAT? YOU DIE FIRST." then he teleports right on above the cheater, doing the "Death From Above" technique and his/her mech loses all of the mech's hp
, because of it, he/she loses some sm credits