This game is still available for download on Vector.JP which is helpful, because only in the description there does it tell you clearing a stage requires downing all six large tentacle enemies. It's a 2D shooter that's a little ungainly; the hit detection and pathing seems wonky, and the smaller enemies can't die from your shots, although they can be knocked down briefly. Arrow keys move and space shoots, and supposedly it's a 9821 exclusive, so it won't run on PC-9801 versions (due to the graphic requirements)