Let me ask so you come up on your enemy who was engage earlier who has a pilot wound but he could still land or even take a ditch. What would you do?
Let him go, and go find someone to fight. No sense in wasting ammo on an assist, and cripples aren't any fun to shoot.
Or let him kill the friendly first, then kill him. I'll do that if I'm fighting someone and a friendly joins in. I just back off and let the friendly get killed.
In all seriousness though, to avoid having more than a couple assists per month all you need to do is not gangbang, and shoot better.