I almost don’t play as Clay (He is level 18 but cards are low level, I have 0% on Clay achievement too), but I noticed this when others plays as him.
Played Escape (The Split), we depended on Clay to clear a path. He is in the middle of the gunfight, but suddenly Guardian stop paying attention when he used his ultimate.
I know some players said that horde was boring and easy, but doing this is not the correct way to increase difficulty. Or at least that the enemies shoot you and when they receive damage stop firing. This feels more smart that anticipating the ultimate

