I have to suggest buying fortifications for rounds (only one round purchased at a time).
Also, I found a few problems. First, when I selected to be a Skill4Life boss, and was reading about my abilities, people were able to damage me from outside.
Second, the fast mover thing has trapped me a few times, and I have found myself smashed against the wall.
Third, when a player on normals has a turret placed and later becomes a boss, this happens.