I am trying with full geared smiter to kill Lilith and she isnt dying at all.
She regain hp like crazy even tho i have poison dmg as well.
I bring her to like 20% of hp and she starts to regain hp to half even tho i keep hiting her with my merc. i mean WTH?
I killed Duriel in 2 secs but lilith cant die. Is a bug?
Poison does not prevent her from healing, she’s immune to this. Ubers are also immune to “Prevent monster heal”, you’ll need open wounds. And of course enough crushing blow to bring her down at last.
What’s your equipment?
Grief, coh, guil face, string belt, yet. LS gloves with ias, exile shield eth, rare rings and amu with LS and resists. Only i have so far.
I destroyed the other 2 but not Lilith. I was hiting her over 5m till my gear broke. And didnt die.
Is my first time i face that and i play the game since ever.
That sounds odd indeed, although you should try with an open wounds item (draculs, duress, gore riders). Tbh I’ve never done it without any form of open wounds, so I can’t really tell the difference 
Yeah sadly i have no draguls or gore rider, would be nice to find them cause i am on Single player. But duress can be done tho i will lose resists and dmg reduction but can test it for Lilith only.