You need to learn how to punish his moves.
A quick search on youtube and I found this video that describes his mechanics and some of the punishes: https://www.youtube.com/watch?v=tjtojCl8DRY . It is missing some punishes though.
Some things missing from that video include:
Punishing his charge-up slash by attacking the second charge-up (either with melee or with a firaga) and then blocking/rolling his reprisal, then airstepping to him. Applies to phase 1 and 2, although phase 2 is unblockable so you must roll (so therefore you shouldn't use firaga and should melee because timing the fireball and dodge separately is trickier).
Getting a full combo in at the start of his DM after he does red-red-blue-blue.
It also doesn't visually show a few of the punishes which is a bit unfortunate as the timings can be tricky to learn.
Otherwise it looks comprehensive enough to get you started on making actual progress on the fight.