Sins you are going Medic most, if you have the über-charge but did not used on your team when they win the round. Then use the über-charge on yourself and your Medic friend when your team start to killing each other.
Fun-fact: If you kill one of your teammate you get minus points which also the same goes for everyone else on your team, so if they want to be on the top of the high-score board then they have to stop killing there own team.
Friendly Fire is a wonderful thing to be caught in. And it's bound to be hilarious as hell. Tell me, have you ever played Arena mode with Friendly Fire and then accidentally kill the only Medic on your team? I know I have.