How to make an event trigger after multiple enemies are defeated

[AlphaBlue1011]

Say, for example, I want to make something similar to TTYD's Pit of 100 Trials. In that, the way to the next room is only revealed after all the enemies in a room are defeated. In SMBX2, making an event trigger after defeating one specific enemy is simple enough, being done entirely in the editor. but how would I go about making an event trigger once a few specific enemies are defeated?

Added in 4 minutes 5 seconds:
I say "specific" because my plan is to separate a section into multiple rooms using MrDoubleA's customCamera, so making it so all enemies in a section having to be defeated to proceed wouldn't work. It would have to be specific enemies.

[mariobrigade2018]

All you need to do is put all of the NPC in a layer, and move all the npcs that use the death event trigger to use the “layer is empty” trigger. However, some NPCs, notably the snifits, don’t always trigger it, which this fixes:
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