That's a good question. Last we saw, he couldn't copy Fine's fireball anymore...Actually, how is he copying these weapon skills?
He isn't copying the weapon skills, since he doesn't have any rings on. I think he just trained really hard and can predict and dodge their moves, while using their own buffs?? We see against the ninja he was able to fight her at super speeds. But I don't really see how it'd be possible for him to be able to keep up with the twins, since he should be moving at literally half their speed?
But yeah, him using fireball makes zero sense?? Either one of his opponents also has the skill, which would be pretty contrived, or he somehow has a piece of gear equipped that lets him use a skill? The latter of which seems like a very odd thing to have had never appeared yet, if it existed. Incredibly strange they didn't address it, but I guess it's all part of the guise that he can still copy his ally's skills, and the trick he's using will be revealed when his opponents figure it out
Maybe a custom gear that looks like the fireball skill? Similar to the pistol his last opponent had?