Complete all three Survival Mode Arenas. You can do these in solo or co-op and on any difficulty so obviously choose Easy. On the first map Arena A, make sure the mine carts don't reach the structures at bottom left and bottom right. If they are destroyed, you will fail the mission. On Arena B, the only way you can fail is by losing all your lives. Keep in mind that if any player loses all of his/her lives, the game ends regardless! Finally, for Arena C, you will need to make sure the razor blade robot enemies do not fully cut down the trees in the bottom middle of the map. If they fully destroy all three, it's game over. Make sure you always kill these enemies as soon as you see them on screen.