Before entering the arena take your creatures out of their cryopods (if you use them) because you can't use cryopods in the fight. I found this out the hard way and almost lost all of my boss fighting Dino's. Fortunately if you need to escape the fight once it's started you can leave the game and rejoin and it will place you and your tames at the obelisk (doesn't give summon items back).

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