Before getting into this achievement, after beating stage 10-5, Deadly Option, SAVE YOUR GAME.
By earning this achievement, it immediately kicks your file into New Game+.
To get the alternate ending, you must complete stage 10-6, Boiling Point, using only a solo level 500+ Valvatorez. However, this is the easy part; this changes 10-7, Defying God, from a boss fight with a level 120 boss, to four fused monsters at level 1000 each.
I would recommend doing this on a New Game+ file, as you will need it to unlock the Android class needed for Netherworld Job Security. Before starting over, you can also do some Postgame content to give a larger bonus to the Cheat Shop and speed up power leveling. Also, if doing this on a new cycle, make sure to get every treasure chest for Check Under the Cushions.
The best map to powerlevel is Mt. Ordeals 4. A fused Desco can hit all but two gargoyles in one turn, and once you get the fist skill Big Bang, using a fused magichange will upgrade the area to hit every gargoyle.