People are still complaining about this?
The solution is easy, if you get mushroomed, spam harness magicka until you make it to the cleansing pool.
Cleanse.
Resume round.
I've never had issues here because of mushrooms. Moment I'm poisoned, it's spam harness magicka and run to pool.
Now, if you end up using your pools because you are reckless around the arena, well, be more careful.
Cool. Forget stamina classes, right. Just use shields.
Easy.
You must be one of those people who don't adapt or change based on challenges in front of you. If an Arena requires a different set-up, then change. Don't sit there and pout because 1 singular set-up doesn't get you through 9 different arenas.