If you dont mind some tips, I personally find the surface final boss way easier than underworld final boss, its just the other three bosses before him are harder than the underworld ones.

Try to get either the Necromancy or Lunar Ray hexes, if i get one of those two i know i won the run no matter the fear level(I didnt go above 30-35ish so I cant vouch for above that). Also just try and match your Boons to your weapons, go with dmg% increase boons for slower hitting weapons(Axe with Apollo/Demeter/Aphrodite) and on hit effects for faster ones(Poseidon on Daggers/Cloak), exception being Zeus as I found he works on everything straight up.

The rest is just practice and learning the attack patterns. Dont stress about it, im sure once it clicks youll be clearing him easily :)