don't go near the sorrow himself, just be patient. I'm assuming you've been a bit trigger happy, and have been killing many grunts. that's gonna cost you cuz it just means this particular fight is going to be long.

So just wait, let him send out the ghosts and wait until your at the end. you will see a REAL dead body underneath a tree, and the sorrow hovering above it. Thats the sign that this fight is over, just go to the body, and the fight will end and you will wake up down river.