The final stage may be short, but it's considerably difficult without a powerful weapon. Head to the right and destroy the two item capsules that fly by. The best way to shoot both of them is to destroy the bottom one by firing downward, and then destroy the other one by firing diagonally at it. You'll get a Machine gun and a Barrier for your effort. You'll reach the guardian of the alien lair, an alien mouth that extends from the ceiling, releasing Floaters on the screen. The easiest way to defeat it is to stand right against the wall and fire diagonally at it. Your shots should also destroy any of the Floaters that appear.
Eventually the mouth and the wall will blow up, allowing you to continue. The next area will be difficult without the Spread gun. Continue to the right and you'll encounter four mouths on the ceiling. When you come up to the first two don't move any further to the right until you destroy them and the spores they release. Once the first two mouths are destroyed, move on to the next two, then jump to the little ledge. At this point, a final item capsule will fly by. You can destroy it by firing straight ahead, and it contains a Spread gun.
Continue on your rampage by destroying the two mouths to the right, then jump to the next ledge and destroy the row of mouths along the bottom. Jump up to the higher ledge ahead and destroy the two mouths level with your gun. Then fall to the bottom ledge again and destroy the two sets of mouths above and below you. Jump over the two holes and destroy the next set of mouths at the bottom, then the ones at the top. Around this time, spider-like Crawlers will appear from the right. They move quickly so watch out for them. Continue to the right and you'll see one last mouth. Destroy it and you'll reach the final boss of the game.
Boss: Red Falcon
The target of this boss battle has no way to counter-attack, but it is very well defended, with sets of cocoons releasing Crawlers on the floor and ceiling around the heart. Destroy the Mouth near the boss if you haven’t already. Then crouch and fire at the two cocoons at the base of the heart. Crawlers are released periodically from the cocoons. Those that appear from the two cocoons at the top will try to drop down on you so pay attention above you. Once the bottom two pods are gone, stand right against the heart (it can't harm you) and fire diagonally at it. Destroy the Crawlers as they appear and continue to shoot the heart. After enough shots, the heart will explode, and you will be shown the ending to the game. Congratulations.