I found that building a tall pillar on the ground, just the right height to let the monster fall down and drop a lot of blood and will not destroy the door.

Or the monster is born with a slow landing,

what 🤔

Is it possible to make changes to this situation? For example so that they can land slowly.

Ah, yeah. I should either apply slow fall to them or search for the ground and spawn them there.

As a stop-gap you could pre-apply slow falling to all the entities, but it would be a bit cumbersome.

Entities will no longer spawn midair starting in 2.1.5

this fix may have caused a new issue, when i make a gate mobs spawn at all y levels no matter the distance, mobs are never all on the same level when they spawn making every wave scatter to the lowest and highest y levels, its un playable.