I’m not a PvP expert, but this is what I like to do. I always mark healers with O and X on my team. It's useful to see where the healers are at a glance to see if I'm running out of range of their heals. I then usually mark enemy to attack as 1. Usually whoever has most medals on the enemy team will get marked as 1. Ideally everyone should attack the marked player, but doesn’t happen all the time.