This trophy is quite a grind, though not the worst one. To make things easier and faster, you should find and use some AFK method, since after completing the game, its possible you wont be even close to this number. My method was at last level of campaign (Cave). I played with one friend (2-people) with 4 controllers. The tactic involves only the level beginning. Put 2 non-playable characters few meters behind the "gate" which separates the starter room. These players works as a bait for earlier steps and as a magnet for more enemies (I think more players --> more spawns, but not 100 sure). Once these two players are in place, continue moving with playable characters (2 which you control) and once first enemies spawn, turn back and get into starter room. Now the harder part of the grind starts. Your goal is to keep fighting the horde and stay alive until you make enough coins for upgrades. I personally use Laser Gun, because its last upgrade is great for this method. So keep making money (both of you), these two players should die very often and they also often ress themselves. While they are alive, they take the heat off, so its easier. Once you finally get the last upgrade and has Laser Beam, ur practically ready for the last step. Now your goal is to block the "gate" with these laser guns so no enemy can enter. Its good if one player (the main one - Player 1) has the beam slightly to the right and the second player slightly to the left, so the beams form a cross. That way its almost certain that no enemy will survive and also majority of kills will get Player 1 (since trophies in this game are completed only by Player 1 stats). Now just fix R2 button so it keeps shooting and you can go AFK. It should hold for several hours, maybe even whole night if you are lucky. I died while using this method twice, but these two attempts were enough for this trophy and also for the biggest grind, aquiring 25m coins.