He's fast, but he has a pretty simple pattern: slash, slash, pause, shoot. What you'll want to do is dash away from his two slashes and then chip away at him as he pauses to shoot. I didn't have deflect either, but I did have absorb, so what I would do is dash into his shots and return fire with the rifle. If you don't have him destroy his own guys, what you'll want to do when he releases them is the same thing as usual, but go after them instead of him. He'll only summon other guys once.
Eventually, he'll switch up his pattern to three slashes, pause, and shoot. Keep going with the same strategy, but just be ready to dodge three slashes instead of two.