My strategy was to keep distance and only bait his air attack (while blocking everything) and blocking it because it doesn't cost much ki (initially I thought it was unblockable though and only tried to avoid it), then hit thrice and retreat. Repeat until dark realm. For dark realm, if the boss started flying and do the projectile + grab/burst attack, I waited until projectiles approached me and then dodged twice to side to avoid everything. When he was on ground, I just kept running circles through the arena until dark realm ended. In second normal phase onwards, he'd do the wide projectile attack almost directly after baiting the air attack and punishing it, so I hitted him twice, retreat and block the projectile. Repeat until win.
Took me a while to figure it out (and more than 10 deaths), but it made the whole fight very simple.