Cariga is in the vault. He can't get in without a key. How can he get the key?

Secondly, there are several small BOSS in the nightmare. If it's just to upgrade, I suggest killing all the small BOSS, and counting the mobs inside, those mobs are more experienced! If you need weapons, give up all small BOSS, only remove the monsters on the road, don't fight the rest, go directly to Cariga, and then slam the key to open the yellow weapon!