Can't get hammer quest item in Moltern Vault

I died to one of the silly one-shot pillars in the arena right before the boss died. Now I can't enter the boss arena and so I can't pick it up. I have seen many other people with the same or similar issues, and while I know that I can fix this by restarting the map, but there must be a more accommodating solution since re-doing these huge maze-like maps is a massive slog.