Invasion Mode (under Splitscreen on the main menu) is great for playing with your friends, but it can also be used to earn a plethora of Credits. Here's a great strategy for doing so (you'll earn about 30,000 credits per hour).

You'll need the following: Widowmaker Turret; Widowmaker Turret specialization; A secondary Weapon you can use to take out the enemies (the MP5 or HVS).

Recommended: Metal Legs, Quickness, Frag Grenades, Improved Ballistics Damage.

Basically, this strategy is about finding a corner where none of the enemies can spawn directly next to you in, setting up your automatic turret and picking off enemies easily. It works best in Siberia, but can be applied to any map. Just find a spot on the map with the following: No Enemy Spawns directly next to it; A safespot you can duck into when you get low on Health; Cover; A clear angle on enemies without to much stuff in the way, as flat as possible.

There's one corner of Siberia down a hill that's great for this, the Turret automatically takes down enemies right as they spawn. Just let it fire and use your secondary weapon to take down the ones it's not firing at. Grab an enemy SCAR or Hive Cannon at some point in the match to use, be sure to chase after the Big coins (1,000 coins is what you get from an average 10 minute WiFi match) and bonus caches. This strategy will EASILY get you through the first 3 or 4 waves, then it gets tougher. Be sure to watch out for the Robotic Dogs attacking your turret, Skimmers (the flying guys, your turret can't take those out), anything with Explosives, especially those SMAW guys), and the suicide bombers. This should get you a quick 10k-30k credits if executed correctly.