ToME: the Tales of Maj'Eyal

Alright... never again in beta 20...
Level 40 Yeek Mindslayer enters a dark crypt, killing everything in his path including some orc boss that didn't last more than three rounds... and then the Doomed Shade of Fluffy appears. With Fluffy's shields... and auras... and reshaped Star/Truth/elemental stralite mace of massacre... and two regen infusions (500/600 hps).
Pumping buffs and regen... five turns at max health plus three more turns taking roughly 300 damage per round... and examining the Shade reveals that it's still at 90% health. Use Superhuman Leap to get away then regen... and the Shade is back at 100% health (not mentioning that Shades seem to have hidden abilities like Increase Life so it was in fact 150% health).
Repeat once... twice... and again... and again.
Almost one hour has passed now, still in stalemate. What to do???
Trying different items in inventory... replacing Wild infusion by Frozen Spear rune for (said) 500 dam... useless, the real damage dealt is roughly 100, which is immediately wiped by regen. Replacing cloak, amulet for seemingly better stuff... nothing seemed to work.
Last hope... replacing the Star by a slime-covered stralite dagger. Much less damage, APR and crits... but the slowing effect could help. And it did...
I'm now watching my clock and I've spent one hour trying to kill that Shade...

I had the same problem with my Ghoul Cursed.

My solution: Rod of Recall, then try again later. (It took me two more tries to finally beat it.)

All your damage is currently split up into small increments between your weapons. This won't get past your shields since they act as direct reductions after resists.

To kill your shade, aka to kill yourself, you need fewer attacks that do drastically more damage. A good idea would also be to stack +% all damage, you might be able to stack +% all damage enough to make the little amounts of damage actually get through. I ended up stacking huge amounts of +% physical damage and just finding the biggest weapon with the highest APR I could to kill my shade with a mindslayer.

My anorithil managed to slay her shade via liberal use of the Rune of the Rift. While the shade was rifted, she couldn't heal herself, and it gave my character a chance to heal up and get ready for when the shade popped back out.