Barrier Knights. First one in one of the Hive rooms, second one in the hallway before the main area, and third one on the upstairs platform.
Unstoppable Ogre, across the gap in the main area.
Fire Pit – When defeated, Acolytes spawn fire pools that cause damage over time.
You have to shoot three runes in the first section in order to progress to the main room. There are a number of spawns in the boss area before the boss appears.