Every time I attack the Sovereign, I use the harp and he attacks me, does a huge knockback and I die. What am I doing wrong?
The knockback is a curable trauma. Handcrafted trauma cures should work, if you don't have any self-cure abilities. The trauma lands when he says "Get off my throne" or something to that effect.
No it’s the boss after that one. Keep pushing and killing the adds as fast as possible with that one I think.