Army of Ruin
221 Achievements
GOG
Crimson Flask
Kill 3000 enemies
42.7%
How to unlock the Crimson Flask achievement in Army of Ruin - Definitive Guide
This is cumulative across all stages, even if you die on them. This will actually unlock very early in your time with this game, and you'll be routinely killing more than 3000 enemies per stage you play.