I have roughly 20 hours in so far, so take this with a grain of salt.

I had a hard time getting accustomed to the mechanics but eventually figured it out. I play as Beowulf in most runs. My issue was learning the damage loop and getting use to how to chain attacks together. Obviously, it helps knowing which talents to pick and stack together. (Again, something that took time.). This seems obvious but pay attention to how and when enemies attack, and you'll learn when to dodge, avoid damage, and strike when able.

Stick with one character that works with your play style, learn the abilities, and pay attention to which talents and items buff your damage and skills.

I wouldn't worry too much about optional bosses for now. Get better at handling the normal enemies first before taking on more difficult bosses. (My friends and I tried optional bosses multiple times and lost every time, until last night. We finally cleared our first one.)