This trophy can be a pain, but Ive found quite an easy way how to this. In my method, I played 2-man coop in Survival - Hospital level. FIrst play normally and obtain some basic weapon. It is really up to you to decide, but I recommend for this method Launcher, Plasma Gun or Shotgun. Once you have a weapon, find the area with "elevated platform" - its very narrow and forms a letter L. Get to the end of this platform and cover the corner - its very narrow so it really slow downs enemies and in 2-man coop its quite easy to hold them back. Be sure to use Player 2 as the one closer to the enemies, because trophies can be earned just by Player 1 (it means as long as Player 1 keeps alive, other players can die at free will and not void the trophy). If you get overrun, just jump from platform to the right, train enemies so you are safe and get back into place. Big monsters can be a threat if you dont have enough firepower, but guns like shotgun or launcher can knock them back and give you more time and space. Very powerful tactic is have Player 1 stand in the back of the elevated alley with Launcher and keep killing large groups from back and player 2 defendind player 1 with shotgun or plasma gun (or any other). For this trophy, you need to do this 6 times.