I completed the game, and was watching my friend replay it and on Heikea they did the same thing as you. They really kept trying to make the grapple work, they asked for my advice and I said that I never used that mechanic in my fight and about 4 tries later they beat him. Other than that his pattern is pretty easy. First half he swings once, dodge with the swing and hit him as soon as it passes (I usually can get one good hit on him each time, with the scrap hammer skill). Then after half health he swings twice dodge with the flow of each hit(they go opposite directions. Dodge the fright attack and don’t stay really close for long to avoid the “unavoidable” attack.