I swarm characters that run ahead of the party.
If a character is ahead of the rest of the party, and I have melee combat enemies, that character is getting swarmed and possibly brought down because I keep enemies on an adversary once they've attacked it unless another character has proven they're more of a threat.
On one hand, I think that's what they should do. Concentrate attacks until the adversary is down.
On the other hand , I'm discouraging heroism.