With almost 10-15k in all stats, I can't even enter the boat for the dlc3 as the winter witch and her knight end up one-shotting me. Am I missing something, or just super under leveled?
You can technically beat them, but it's so fucking hard to fight those enemies that I nearly never bother. You basically need to get into boss slaying mode, equip all sorts of fucking gear, and then try. I have beaten this game like 5 times, maxxed out my level, chugged Souls of the Outer Ones, and I struggle to beat them consistently on Difficulty 0.
The biggest issue is the frozen magic bullets. Getting frozen will stop you from reacting. Buffs are useless because of dispel, so you need to capitalize and try to take down the Winter Witch as fast as possible. The Knight does a lot of damage to you and can stun you, so you need to build stun resist too.
You could try starting with the Andor Sword and then use equip swap to switch to the Deep Sea Knight Anchor and try to hit the witch with a black slash then swap to Guardian Bunker to tank the hit from the knight, but if you get frozen, you're basically dead.