When you get into the boss room, to your left is a row of grave stones. Immediately dodge as soon as you enter (the boss likes to shoot you nice you and early), then run to this grave stones. If you're fast, you can be on the side closest to the door, and the boss will position himself on the other side. From here, you can do quick attacks or heavy attacks and mostly be safe from any kind of stun locking. Always keep your health at full, because his heavy attack, if it lands, will probably kill you. Don't worry if the boss comes around to your side, just move around to his. Just keep healing and slashing away and you'll be in second phase in no time.
At this point, you need to parry. You can get stun locked reeeeeaalll easy when the boss is in this mode, and the first thing he does is bust up those gravestones. If you have a lot of bullets, I recommend spamming them unless you have gotten the parry time down perfect. One of them will probably hit. You can do one of two things after you hit the boss -- you can try for a second parry, or, if you have a powerful weapon (I didn't, used the cane) you can probably run up and finish him off. I'd recommend the second parry to be safe.
If you have him down to a sliver but things are getting dicey, toss some projectiles at him to finish him off.
Might take a few tries with this strat, but it lets you easily focus on getting your timing down for parry in second phase without any danger of dying in the first phase.