Bonus coffers with mobs guarding them should be removed from the game

[WellFooled]

Skip them, then after everyone exits, run back and tag them all.

This is a strategy I use to good results. Even with monsters guarding them they can't kill you before you open the chest and receive the contents. You suffer a little armor damage, but worth it if you're looking for a particularly item because if it drops its yours uncontested.