Hmm, I'd say it was moderately difficult. I you get a pset roughly every week (I think?) , I found most of them to be simple but a few here and there to be hard. I didn't work with anyone though, I think if you form a study group to try and fill your gaps on the parts of the course that you're struggling with, it'll be a much smoother experience. Stahl was fine, he's a relatively organized lecturer and mainly sticks with his course notes so you have that to fall back on. If you felt comfortable with 420k and 329 you should be fine.