Alright, so this dude has like five or so attacks and the big thing is that around 50-60% he'll split into two. I'm sure you already familiar with this. As a melee, and more so without a shield, you basically need to get a feel for these attacks (timing, ranges) so that you can feel somewhat comfortable even when two of them are coming at you. Positioning as far as I can tell is not very important, just know how to react to each attack.
When it comes to dealing with two of them you always want some stamina on hand, you always want to above 70% health (if not full, even) at all times, you want to be extra careful where you heal (don't underestimate the slower, easier moves to dodge), and always play it very safe (even at the very end, because I almost died finishing it with a double hard attack lol). Perhaps stuff you might already know from fighting multiple enemies, but it is especially true here.
First there's the real scary one, where he summons two black balls into the air and a very powerful, explosive projectile fires out of one them randomly (IIRC). For this one, it's a drop everything and run moment (running as opposed to rolling is actually pretty effective against some of the projectiles), especially when there's two of them out (so even if you think you can get free hits on the other, I wouldn't test it). The hard part, aside from the massive damage, is how large the explosion range is (making the small projectile deceptively huge). There's a bit of a sound cue when it gets close, so that might help dodging, but never be conservative when it comes to avoiding it. I've never seen double up on this one, but god forbid if he does lol.
Then has a basic soul arrow move that's pretty easy to avoid. When he's alone, this is a like a freebie. In a pair, it is just another reason why you want to save your stamina, even after an attack. He seems to always start with this, making it nice if you want to apply some DEF item or resin.
The sword swipe (and follow-up) is not much different, mainly once you reached the point where you dodge it without thinking, but it isn't to be underestimated in a pair. This attack is actually pivotal, because it seems like when there's two of them, they seem more eager to use it. Once you've dodged it and know the follow-up can't reach you AND you have a good grasp on what the other one is doing, this is your best moment to attack. But again, don't get cocky.
There's also that explosion around him. You see it coming because he arches backwards while crossing his arms. This is only worth panicking about if you are really close (I didn't bother with my caetus partially because of it), otherwise it can actually be really handy for isolating one of them. Although it's rare, this one will ruin your day if you completely forget about it.
The move where he summons a big fireball over his head and then does a wave of three fireballs is probably his second scariest move, but rather manageable (just less so when the other one is trying to cut you down). These don't seem to home at all and once you become confident on where to dodge/step to avoid them, you actually have a bit of a breather you can use to attack the other one (but first you have to consider what the situation with that one is).
Finally, there's the laser beam. This is probably his worst attack, so it's great when he does it when there's two. Once he shoots the laser, just try to stay far away from where it landed or directly under him. You can use this to focus on the other one or hit the laser one while it is coming down.
One thing to keep in mind is to keep your estus nearly full when you reach the boss (which isn't something you don't have to worry about as much compared to previous Souls games, but here it is somewhat the case). I used the forest ruins entrance, because its the easiest and shortest of the three. The bonfire is actually pretty close if you sprint. Also if you plan on using a shield, you probably want to use one with a lot of Dark DEF. Likewise, equip a dark ring, eat that dark def item. Maybe I'm wrong actually, but that's what I did. I also used lightning resin, since I figured he'd be weak to that. Again, don't actually know. You may also want to figure to buy out some NPCs who sell effigies lol. The blue dude in the undead crypt sells a bunch.
Hope I didn't forget anything.