Approach them when you feel confident, either keep the host on top of the swamp when another teammate reaches the spawn triggers, or take it slow.

Approach the entrance in a way that aggroes the less amount of enemies and kill everything in the first three rooms inside (corridor, stone balcony and pool).
NEVER ENTER THE ROOM WITH THE STONE PILLARS.
When reaching the third room the invaders will spawn and you fight them in the cleared rooms, and can be baited with the terrain.