Unless they've changed it, enemies get stronger when you have multiple people. The draugr's are no joke, especially the bow ones if you get a 1 or 2 star one, they can pop you with a lot of damage out of the blue.

As others have said, better food is top priority. Shield that can parry is another. Potions actually start becoming important in the swamp. The poison resistance mead is a life saver, pop one once you get into the swamp to not be farted to death.

Another thing is, because you're playing with a second player; Communication is really important. I play with Mrs Whamy, and when we get to the swamp (or later) stages, we really get our game faces on and make sure to call out anything dangerous. That's the thing that kept us alive the best.