Sir Ambrojoe Achievement in Vampire Survivors

This is cumulative across all runs.

Stage Killers only spawn in the last few minutes of Stage 5, Cappella Magna. They are the enemies that look like all of the main starting characters you can play as. It's possible but unlikely you'll get this many kills of these enemies in a single run of this stage, so plan on a solid two (that's how many it took me) runs to the last minute or so of Cappella Magna to get enough cumulative kills on these enemies to unlock this achievement.