Beat up to and including level 10 in Bloody Palace mode. To unlock Bloody Palace mode, beat the game on Devil Hunter difficulty. Do not quite the game, you need to die or finnish all 101 floors to get the achievement. Below are a list of the enemies from waves 1-10:
1 Scarecrow x 3
2 Scarecrow x 4
3 Scarecrow x 6
4 Frost x 2
5 Bianco Angelo x2
6 Mephisto x 3
7 Gladius x 6
8 Chimera Seed x 9
9 Chimera Seed x 9, Scarecrow x3
10 Scarecrow x 12