This comes from my experience in Uncharted 3's survival, and the fact that archers refuse to step inside the zone, even if you are about to lose it.
Although, it's not only an archer problem, I understand the benefit of fighting outside the zone, but another thing is to ignore them completely.
How come I'm holding a zone by myself, while the other three players lose the remaining two, stay fighting the lost zones, and if I ever get downed, we pretty much lost the game.
So, my proposal is to increase the size of the circle, right now is too small to have two players fighting a whole wave. But keep a counter, every enemy has to be killed inside the zone. Otherwise they just respawn forever. Or make them invincible until they are inside the circle (like it works right now for the ranged last hit card).
Thoughts?