I spent most of my retries trying to keep Luciatel and my last Loyce Knight alive. That honestly was the hardest thing about this boss fight for me. (Still, get all four Loyce Knights. It helps. A lot.)
1. Don't fat roll. You die if you do.
2. Target Charred Loyce Knights first, the pyromancers can make you into a sitting duck if hit.
3. Phantoms are friends here. He doesn't get too much of a stat boost, and meat shields are great.
4. Lifegems, not Estus. Estes heals more faster, but gets you agroed. He tends to not agro when Lifegems are crushed.
5. The only real dangerous attack is his stab and throw. His slashes are easy to dodge, and his crystal AoE can be seen from a mile away.
6. While heavier weapons are more viable here than say, Fume Knight, I would prefer a Greatsword at the heaviest.
7. Be prepared for him to switch agro randomly. He does it a lot.
8. Don't be afraid to hit him more than once, but keep an eye on your stamina consumption. It will be needed to roll.
9. If you summon Twiggy Shei, he almost always agros in him for some reason. (Out of all ten fights with him, trying to keep Luciatel and Loyce Knight (or LK) alive, he practically mauled Twiggy, and ignored all of us smacking him with weapons.)