Small Arms

Mission Mode is accessible by choosing the option Single Player from the main screen and the selecting Mission Mode.

Basically mission mode will give you 5 lives and pit you against any amounts of enemies from between 1 to 3 at once. There are a total of 12 missions. The thing that makes the mission mode so easy is that you get infinite continues. This means you can screw up on level 11 and lose your last life and then come back with all 5 lives to try level 11 again.

The only level that might give you trouble is the last one. The last level will force you to fight against a slightly larger enemy after killing 3 robots who launch rockets at you. As you fight against this larger robot enemy smaller robots will be attacking you as well. You only need to kill the large robot so focus all of your fire onto him while avoiding the smaller ones.

Mission mode can be done with any character but I recommend the chicken named Pector El Polo. The main reason being that his secondary fire (activated by hitting ) affects a large area. When fighting multiple enemies hitting more area means hitting more enemies.
Of course it can be done with any of them so choose whichever player you want.