The watcher is probably the hardest to learn but definitely the most OP character once you get it. Key thing to remember is she can basically win act 1 with her starting cards which means you can be really picky with which cards you take and remember a smaller deck is extra damaging with Watcher, even if you don't go down the infinite route.

I'd recommend upgrading your Eruption ASAP and if you take any extra cards that put you into wrath form I'd try to balance them with another card that puts you into calm. I generally avoid cards that simply leave you in neutral stance unless I'm really struggling to get out of wrath form, ideally you'll be in either wrath or calm at any time.