The longer it takes you to kill, the more damage you take, as it's over time.
Spoiler
Get better DPS and find other ways to sustain besides leech and regen. Never stand still for any reason. Learn to manually steer the void puddles in the maven fight to the outer border of the arena so you have more "safe" areas to work in as the fight drags on.
Don't get hit by anything ever, in the maven fight. Nearly all the beams for all stages nuke your leech, regen, and on-hit recovery. Just learn to move your ass and not get hit.
As for no-flask cheats, which seem to be the big ultimatum bugaboo, you'll need to add skills to your hotbar that get around having no flasks: warcries, withering step, vaal skills, phase run, etc. Put them on whatever resource you have that you can sustain: if recovering mana is turned off, then put them on blood magic; if all regen is off, then use a vaal skill; if there's no adds or souls and the boss is invulnerable for long periods of time, consider using warcries or taking an ascendancy that gives you adrenaline or charges.
Purchase gear with reduced damage over time mods. Carry it with you. Use Arakaali to help with (especially) chaos dot and ralakesh to help with phys dot. Stack reduced dot pantheon before going into zones with heavy phys or chaos dot. Hit up Summit map to reduce burning damage (as well as ignore burning ground). It costs significant money and resources now to get dot protection. Sorry but it's necessary.
vs this:
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Play better.
Yes, the second one is much better. Image having to design a game for a playerbase that refuses to be creative, grind or do anything but spam right clicks in boss fights.
