I'm not a Gunman myself so unfortunately, I can't name this ability. I've encountered it several times (if not each time) when fighting Duelist NPCs (of course, with dual pistols, lol). It's that move where he stands away a few meters from you (I can't exactly remember if he kicks you away or jumps backwards), shoots a few times from the distance and then performs a horizontal flying jump towards you while shooting many times. A sommersault on the floor follows and then he shoots once more.
I hope this description should do it 

However, the phase where he shoots you a few meters back is the one bugged, because he stands there completely frozen while your character obviously gets hit by the shots the enemy is supposed to perform.
Any thoughts?