So I just fought a suspicious player. I couldn't seem to block any of his attacks and he would just ignore some of my attacks, not dodging, blocking or deflecting them but not getting hit either, they seemed to just go through.
What else was suspicious is that he took forever to load into the game. I was stuck in the VS screen for many minutes each time, and the little class portraits both on my profile and his were black in that screen too.
All this made me wonder if he was using some kind of tool that manipulates his connection.