The key point is that even at level 120, one-shots can still happen for a few important reasons. Having 40 vigor is decent but late-game areas like crumbling farum azula really need closer to 50 or 60 to feel comfortable. Something interesting to consider is how different types of damage work - your armor might be great against physical hits but leave you wide open to stuff like holy damage or magic, so mixing up protection with things like the pearldrake and dragoncrest greatshield talismans makes a huge difference.

Some bosses have these really nasty moves that can bypass normal defenses, like that waterfowl dance or maliketh's attacks with the destined death effect. If you're in the dlc areas, you'll definitely want to grab those scadutree fragments since without them you're taking way more damage than necessary. Tip worth noting - if you've put tons of points into damage stats but skimped on vigor or endurance, you're setting yourself up for trouble especially with those multi-hit combos that'll tear right through you.

Might be worth using a larval tear to respec and balance things out better. Pure caster builds tend to struggle more with surviving since they can't interrupt boss attacks as easily. Getting your vigor up to 60 and grabbing that erdtree's favor +2 will help a ton, and don't forget about the ritual shield talisman for those really brutal first-hit situations.