Infestation mode is a multiplayer co-op mode comparable to horde mode on Gears of War. You can play with up to 4 people online or even by yourself. To make this achievement easier, start a survive custom game and set the difficulty to casual and wait for four people to join and then start the game up. Each wave you survive is recorded so that if you give up or lose you can restart from that wave at any time and still unlock this achievement. The more waves you survive the better the weapons you get and the harder the enemies become. It's relatively easy to beat wave 30 because the weapons you're given on higher levels are insanely powerful
To make this even easier you can keep playing other maps to level your upgrade rings. All of the single player upgrades you have unlocked are carried over to this mode - they just need to be unlocked. I recommend you use the nano rifle throughout this mode as soon as you unlock it as it one hit kills most enemies. Also pick the shell nano suit power because this is very useful compared to the other powers.
If you need boosting partners, refer to this thread.