Molten Vortex has an objective called “Survivor” that says “Win the fight with 6 or less HP.” When I start the fight, the objective list says “100/6,” regardless of my actual HP, and counts down the damage from there. (So if I take 5 damage, it goes to 95/6, for example). The objective seems to be to lose 94 HP over the course of the match, but I haven’t been able to do this yet so I can’t confirm it.
I’ve also noticed that objectives that read “Lose X or more creatures” actually count the number of creatures you kill, not the number you lose.