I think something about this game's balance is screwy. Late game enemies will easily crit for 150% of your maximum health even if you have full upgrades and items and outlevel them.
You can somewhat mitigate this by learning the passive skills which increase your max HP and defense temporarily at the start of each fight and the skill which lets you survive lethal damage once (go to the skill crystal, go to "next page", and then use the cancel button to invest skill points into the passives) but its still a damage race for every fight.