With this trap mobs are automatically killed after they are spawned.

I have built my idea here in the creative.
It is quite easy to replicate.
The glass block in the picture is supposed to represent a monster spawner in a dungeon in survival mode.
After an enemy is spawned, it is led by the current into the shaft where it dies from the fall.

This is extremely effective because depending on the type of monster you get a lot of arrows, bones, Verr. Meat or other gets.

Under the funnels, which are at the bottom of the shaft, are alternating chests and redstone chests.
Through a corridor with ladders you can easily get down to the bottom where you can pick up the items.