Mission 18 Endgame
Most of this mission has you driving a boat with many enemies all around you. You have infinite ammo and don't really need to aim. Just position your boat so that enemies are in front of you and you should be able to get 20 kills before the end of the mission. Reload checkpoints as needed.
Here is a video guide with English commentary!
Here is a playlist of guides for this game!https://www.youtube.com/playlist?list=PL-HeW6XkKdUK97oYVSMU7...