Wacko

The player assumes the role of Kapt'n Krooz'r, a small, green alien within a bubble-topped spaceship. The goal of each level is to eliminate the monsters, accomplished by shooting twin pairs in succession. As the player progresses, shooting monsters out of order creates mutants that must be unmatched before they can be eliminated. Source: Wikipedia, "Wacko", available under the CC-BY-SA License.