After part 3 of the alien spaceship, you encounter the gigantic ostrich-like robot, Ostroid.

How to Defeat Ostroid

Don't get too close to the bullets that Ostroid shoots downward. Jump over the pink orbs that it shoots. If it crouches down, be prepared to jump over the bullets that it shoots horizontally. If you can hit Ostroid rapidly enough, it will go farther and farther toward the east wall. If possible, jump on the tile just east of the center of the room, then step on the switch that appears, and some of the tiles on the east side of the platform will disappear. At that point, try to hit Ostroid rapidly and move east without touching Ostroid to push it into the hole on the east side. If you do so, Ostroid will fall and be destroyed. If not, just keep hitting Ostroid and it will eventually be destroyed.

Moving On

After Ostroid dies, jump north on the tile, then keep jumping north to go through the door. Go north to exit this part of the spaceship. You will proceed to the spaceship maze.