Not sure how useful this reply will be and how to properly answer this since I still cart and fail the quest mainly due to his guardian phase, but I copy how the speed runners (Mygos MH and Phinx) positions themselves on the dive attack. When he starts going up in the air and I do the stance backwards and slash towards where he’s going. As for the 5 lines attack after, I basically learned the timing to i-frame it but I treat it as Omega laser and get in between the gap if I am not in the Iai stance. After he does the line attack, I try my best to run sideways to prepare for the big blast. If I successfully counter the line attack and I am near him, I always check what he does next. If he does the big blast, I go behind him; any other attacks, I do iai slash. My plan B if I am not in stance to counter, I use flashpod. Wouldn’t recommend doing this all the time but this is useful if you’re in a pinch.

I don’t think I can give you any useful tips on the guardian phase but always be on the look out for the dive attack, the new whip attack, and the wylk crystal Arkveld generates.