[Solution] Stunlock
I've always been a fan of this idea in games: diminishing returns.
For those of you who don't know what that means basically, I'll give an example.
With diminishing returns player 1 may stun player 3 for 5 seconds
player 2 comes and stuns player 3 for 5 seconds, but since he has recently been stunned the stun only lasts half as long.
and it keeps going for a period of time, eventually the unit will be immune to stun until a certain amount of time has passed since his first stun.