"Flip" a map twice.

You need to flip a map twice in one match, so you need to get to Wave 25 once so it goes back to Wave 1, then get to 25 again. So technically, Wave 50. The best way to do this is to play a map you are familiar with (preferably one with lots of buildings and cover), and to play solo or with 2 players max. When you play solo or 2 player mode you have a lot more health than you would if you were playing with more players. With 4 players, you die very quickly so playing with many players can make this trophy harder if anything. On solo, you can die once and you get self revived. With multiple players, you can revive each other as much as you like. That is one of the biggest disadvantages of playing solo.