La Pucelle - Level 480 in one turn

A level 1 bat defeats a whole Baal army for mass leveling! This was made possible by merging a lot of iron boots (-100 to most stats) for an item with high negatives, which I used to lower each Baal's HP to 1. Triple EXP (shop wish) was on, and bats have lower EXP requirements per level, so that boosted the level gain even more.