In the first level inside the ship, proceed until you're past the wooden crate walls, which you broke through with your rocket. When ricocheting off walls toward the end of the level (before the boss), there should be a space inside the wall on the right. Jump in and walk in. If you're in the right place, you should find some sort of wooden wall built into the stage wall. Charge into it to break it, and an explosion will show up. Infinite enemies will spawn from the explosions. Maintain a distance from them, face the explosion, and press the attack button repeatedly to kill enemies the second the spawn. Kill enemies until desired score is reached.