13,579 HP still takes 407 (3%) from (Expert) Sloth's DoTs.
considering the damage increased with hp , it seems useless IMO. It;s pointless to have 20k hp (i'm using round numbers to be more clear) if you take 600 damage per turn , instead of taking 300 for 10k hp. From that point of view , it's an equal trade. I'd rather buff resistance if since it makes it's damage miss entirely.
Ofc , it's entirely possible that i'm wrong as well....so feel free to corect me people if i'm missundertstanding the mechanics here
You're basically right.
As far as just getting all the prizes here, you probably don't need to fight demons whose levels are so high that you need +HP to avoid dying instantly to an overdrive. So in that context, you probably want to go with something like what sanahtlig suggested.
But, be prepared to adapt accordingly if your union is at a point where the bulk of your members need the turns but are struggling to survive past 2-3 overdrives, and that you're not on pace to get all the rewards.