Which bosses and skill are one shotting you? The later game bosses do have very high Attack stats, so make sure you're also gearing with Defense and Resistance in mind.
Viewing post in Dungeon of the Devourer comments
But I have late game gear, and some enemy attacks deal 50 dmg and some 26.000.
An example would be the slime Dragon in the depths, even when slime logged one attack dealed 13.000 dmg on resistance. I had to reset the fight for the right RNG about 7 times until i was able to kill it within 2 turns and without being killed. The slime Zealot was similar.