boss battles3 survival mode3 2d shooter game2 alien invasion2 arcade style2 endless levels2 intense gameplay2 online2 retro graphics2 special power-ups2 upgradable weapons2 2d fighter1 action packed1 action-packed1 bullet hell1 bullet-hell1 canvas game1 fast paced1 fast-paced1 fighting game1 game1 gamepad support1 mobile game1 offline game1 player mode1 rpg fighting1 shadow fighter1 single player mode1 touch controls1 versus game1