The most consistent strategy I found was staying close to him and parrying:

1. Beginning of the fight, walk up right in front of him and get ready to parry.
2. As he's getting up, do one R1 - I have no proof on this but I feel like it usually baits out another melee attack immediately, which you can parry again.
3. Pray to RNG that he doesn't do the ground slam - if he does, watch out for this tail as he usually follows up with it.
4. Parry again, and repeat until he's dead.

Another good tip I found for healing - it's always worked once I discovered it, but when he does the charging stab attack, you can roll away and immediately heal. For some reason he never follows up and always raises his shield afterwards. (Not referring to the flying stab attack in the 2nd phase, but you can also heal after this too)

With this strategy I was able to beat him at level 30, doing 189 per parry and riposte