This doesn't sound like a case of playing your character wrong in battle, there really wasn't much you could do at that point with such low AC and HP. Unless I'm misunderstanding it sounds like you ran into his traps in the dungeon and took a lot of damage for it before the fight even started (Don't feel bad, my party did too), that set you on a bad path already. Your even bigger mistake though was not having any healer besides a Paladin with his measly Lay on Hands. You don't need a Cleric or anything but you should have someone invest in the Medicine skill so they can buy a Healer's Kit and use the Treat Wounds action between combat encounters (Once per hour per person), it'll save you a lot of grief, 2e generally assumes that the party is at or near full health before major encounters. My party doesn't have a Cleric or dedicated support either and we would have TPK'd like 5 times in Plaguestone if my Wizard wasn't investing in Medicine