You gain XP by killing aliens. When you get 50 XP you get a skill point, which you use to upgrade stats. Enemies: -Alien. 2 Health. Gives 2 XP -Half-devil. 5 Health. Gives 10 XP -Miniboss. 9 Health. Gives 30 XP -Boss. 12 Health. Gives 50 XP
