Note about the prototype/sample:

The label says it was released on May 20th, 1988, and also indicates that there is a special warp feature that allows you to go to any stage.

 =>Easy Kill: To quickly and easily kill the bosses of levels 2, 4, 6 and 7,
toss a grenade at them. When they begin flashing after being hit, pause the
game. As long as the game is paused, the boss will continue to suffer new
damage from the grenade as it flashes. If you wait long enough, a single
grenade will completely kill a boss.
->Easy Kill Boss 5: While the boss is entering the screen, pummel him with
grenades repeatedly. You should be able to do hefty damage to him quite
quickly.
->Plutonium Boss Tip: When you face the Plutonium Boss, with the level seven
music playing, get him in stuck in the top right corner of the screen and
blast away with you gun and grenades as quickly as possible. You may get
hit a bit, but you should do much more damage to him; also, he generally
won't be able to attack you diagonally while he is stuck in the corner.
* Or, once you beat the big head boss, die. When you reappear, don't move,
and only use your gun (not grenades) to attack. He shouldn't be able to hurt
you.