I honestly can't believe you're right and I suggest double checking that armor. I have a monk with 400k hp and 75% armor (25/25/24/24) and I can take multiple hits. I've also done it with a huntress with the same 75% armor and only about 250k and I'm still able to take a fireball or two before getting worried.
Something here isn't adding up and I'm not trying to be rude, just saying double check your armor stat once inside the match before you start a wave. I've def. taken more than one hit every time so either your armor isn't maxed to get one shotted at 350k hp, or you're getting smoked by multiple fireballs at once somehow. I do highly suggest checking those stats out in the match before a wave rather than in the tavern. I've checked mine in the tavern before and it's showed 75% and in a match before upgrading a new piece, it's under 75%.