Thanks for all your advice Avatar! Those are really good points -- I was stuck in a mindset of "I can cast two level 1 fireballs" when I should have been thinking "I have the HP of a level 3 character and I hit like a truck". And I definitely need to use Entanglement when leveling more. And especially the Swampsouls, I got so used to ignoring my physical attacks that I forgot Half-Dragon + Vine Form's massive HP pool and Half-Dragon's knockback made physical attacks both possible and good.
I guess I really was out of my comfort zone after all. Usually when I'm playing someone I'm comfortable with, I eye up all the monsters three levels higher than me and go "I wonder if I can take this guy". But here my mindset was to just take whatever "good enough" opponents a single level higher than me I could get, even if I could do better.
Took a shot at Endless Blizzard Curious. It seems really cool, and I'll admit I was terrified when I saw how much free stuff it hands you at the start. Love how this run seems, even if I'm thinking my usual purecaster approach won't be so great lategame... but then the game crashed.
EDIT: Actually, I think I have a guess at what went wrong: