You have to defend what remains of human civilization during a campaign that will take you through different levels that are all full of enemies. Luckily, in order to defend yourself from all of the aliens, you will have a few good machine guns that can vaporize hundreds of aliens in a matter of minutes.
It's true that Call of Dead: Duty Trigger 14 doesn't have a wide variety of weapons or enemies (they're all the same kind of alien), but it does have an overwhelmingly large number of them. During the campaign you can annihilate thousands of enemies using just your machine guns and the few grenades you're able to find.
Call of Dead: Duty Trigger 14 is a first person action game and shooter. Though it isn't particularly good and it doesn't have great graphics, its name is funny enough to make it worth checking out.