Defeat 150 enemies in survival mode.
You want to do this in co-op, even if you don't have a partner it's still better for you to control 2 characters by yourself than doing it in single player. Use Jimmy and Jeff, as they are the strongest characters. Pick the Hideout map, it's wider and the visibility is better. Hit enemies with :Xbut: at all times as kicks are too slow, use your special attacks ( + ) only when you're in trouble and don't be afraid to overuse your power attacks ( + ) as soon as you can use one, in fact you should keep pressed down at all times. Always have a special attack ready especially for level 10 and onwards where the bosses starts to show up. If you got a decent partner you will get this without too much trouble but doing this all by yourself will be very tricky.