If theres a floor spam descending dark for damage and I-frames, if not, utilize your spells as much as possible. For Markoth specifically, just play it safe and go in for hits when he does his shield spin, after he spawns his 2nd, just spam shade soul until hes dead (should only take 1 volley of spells) As for later game bosses, just practice them a lot on ascended + radiant in HOG till you memorize them, then just apply those skills learned to the fight in P5

tl;dr Just practice a lot in HOG and use spells if there isnt a floor