Radec sniper spammers. Making my main look cheap and terrible, hate them, always target them specifically, dont care if i dont win, i just want to make sure they suffer.
If your partner disconnects early (even before the fight), or was subjected to Kat KCs or something, it gets annoying. It's REALLY not fun getting combo'd in-between two Kratos' or two faraway Coles just spamming or things of that nature. Or a combo team where one keeps you busy and the other is PaRappa or Emmett getting AP.
Maybe it's just me being to more fair person that I am. Unless that person has a 4+ life clearing Level 3 and they're halfway there from Level 2, I stay on the side until either my partner loses all their lives (in stock), they win, or the opponent provokes me and tries to get "free" AP from me.