Cooperative play :
If you cannot work with your team to manage horde attacks, then you probably shouldn't be playing the game in co-op mode.

Don't blame me if your silly ass wanders away from the group, and you get mobbed by a horde! (a.k.a. - DUMB-ASS )

Strategic Awareness :
Simon Says, "Stay close together"
Simon Says, "Cover your teammates"
Simon Says, "Constantly evaluate cover positions"
Simon Says, "Communicate constantly"
...Now, run out and engage that incoming tank or horde...
D'oh! Simon did NOT SAY, "Do something stupid !!!"