That's weird, you shouldn't be one shotted on normal (aside from some very specific places like the big room o' death in the armoury) (or unless you're meleeing people as a yeek possessor or something). Dangers should be status effects like stun, freeze, etc.
Stuff that tends to one-shot you in TOME, even on normal difficulty:
Archers (pure archer build classes and not hybrids using bows) - when you have low armor
Necromancers since the necromancer update - when you have low magic save and <50% cold resistance
All defiler-type enemies: Elven blood mages and Worm-That-Walks being the most common one-shotters if you don't have a high blight resistance (and preferrably also crit damage reduction or crit shrug off chance)
Lategame melee berserker type enemies, including non-rares - they usually have the attack which deals 150+450% weapon damage which is guaranteed death on most characters if you have low armor without absurd damage shield
As for bosses, from my experience the biggest early game offenders were the Minotaur variant (Horned Horror or whatever its name was that could proc a 3x lightning skill from attacking with his brawler skill 3 times in a row which is a one-shot on most characters), Trog (when low armor), Tempest guy due to massive lightning damage variance, Celia cuz same necromancer problem post-update
Lategame any boss has a potential to one or two-shot you (save for a few that really lack high-damage attacks) but I'd give a special mention to the Spectre of Telos whose cold damage nukes are on average like 3x stronger than average boss nuke damage.
However for new players I'd say the biggest threat overall are BULWARK type physical enemies (of the rare/unique monster variety because regularly generated bulwarks deal basically no damage), since it's extremely easy to get put into a "counter" state where you take double damage and then get hit for Overpower or whatever other strong physical attack the base monster type has for a quick one-shot on non-tanky characters.