the lower area there had one Corathus mob and as you go towards the back there is a ramp to a room with one big mushroom in the middle.
There are 3 mobs on the left as you come up the ramp. after clearing the room many times The first repop of those 3 was the infector for me. I can't really say which mob was the placeholder if there really is any. You seem to get random pops there.
I did get Nogglegrop Fendlebar the Unwilling to drop while camping the legs.
Splitting the group of 3 with my necro took me a while to figure out. The best way was just root one and snare the other and run back to the lower room or across the lake to the zoneline. I was not KOS to anything on my trip to the zoneline.
Once I had 2 I could root one pretty easy and kite.
THose 3 mobs are not the only ones in the room. Just the first ones you see and probably pull. They tend to come in groups if you can'y pacify.