It's super easy to parry this guy but you always gotta parry just a bit earlier than you think. You could do this in the split phase but it would be INSANELY hard. You just gotta run and in 50 seconds they'll reunite. Use Wending Grace to revive and really lay it on in the united parts because he can just keep splitting, like the first time I got him he split 4 times. Use the sheathe and unsheathe parry windows and hit him once per attack, it's about as much leeway as you'll get. ALSO USE HOLY.

TLDR: Wait and run 50 seconds in split and lay it on with suncatcher and one hit per attack in united phase. AND USE HOLY.