Chapter 2 - Academy Hallways - Easy
A short ways into the level, you'll come to a bottomless pit with two or three wooden platforms suspended above it. There's a checkpoint on the other side. Hit the checkpoint, then walk back and off the edge into the bottomless pit. Press the LB/RB button to switch to a different character, which will spawn at the checkpoint and also revive the character that died. Repeat this 24 times, and complete the level as normal, at which point the achievement will pop.