The waterfall stage is the only stage which scrolls vertically. Your objective is to reach the top. In general, you will always jump up, but you should avoid the sides where rocks occasionally fall out of the small holes in the cliffside. The first weapon sensor that you encounter on your way up will contain a Fireball gun. Another weapon sensor which contains a Laser gun will appear a short distance later. Shoot the sniper waiting just above, on the far right.
The Scuba soldier in the water is difficult to kill unless he is standing up, so ignore him. Higher up, there's a turret in the top right corner. Quickly jump to the upper right ledge and destroy the turret from either the bottom, or the left if you have the Spread gun. If the turret activates before you get into position, you’ll have to destroy it from the bottom, where soldiers appear from the right. Watch out for them. Further along, there’s a long bridge connecting the ledges on the left and right sides. Two small flames move back and forth across the bridge which can kill your player.
Two weapon capsules will fly up from the bottom of the screen when you reach the bridge. One of them contains a Rapid fire upgrade, while the other one provides a Barrier. Be aware of the turret above you, which will activate wen you get high enough. You can easily destroy it with a Spread gun. Otherwise, you have to jump up to its height, and alternate between crouching and standing up to fire at it until it’s destroyed. Remain standing under the ledge above so that soldiers don’t leap down to attack you. After the turret is destroyed, continue jumping up until you see the floating ledge. Jump up to activate the turret nearby and destroy it. Then jump onto the floatign ledge, and beyond to the right ledge. Shoot the Scuba soldier to the left and continue. If you don’t remove him, watch out for his mortar and the pieces that break apart.
Ignore the Scuba soldier on the right and continue. There’s a Spread gun in the weapon sensor above. Collect it if you need it. Destroy the turret above you and then jump up to cause the screen to scroll upwards. Destroy the turret on the long ledge quickly because soldiers will start coming in from the right to attack you. Leap up again to activate the two turrets above you and then destroy them from below. If you don’t have the Spread gun at this point, it will take a little more effort to remove them.
Watch out for soldiers above you. When the coast is clear, carefully make your way up to the ledge above and jump onto the floating ledge on your right. The next weapon sensor contains a Machine gun. Continue jumping up and shoot the Sniper waiting on the left ledge above, then keep climbing to reach the top of the mountain, where the entrance to the second base can be found.
Boss: Mechanical alien
As soon as you arrive, move to a corner and wait for the arms to appear. Once both arms are moving around, they will shoot fireballs from their hands roughly every eight seconds. The head will also shoot out three fireballs from its mouth to the left, center, and right at slower intervals. Concentrate all of your firepower on one arm to destroy it as quickly as possible, but take time to dodge any fireballs that approach. Then fire at the second arm until it is destroyed. Next, stand slightly to either side of the head and start plugging the mouth with bullets. In the right location, you should be able to shoot its mouth without needing to fear collision with the fireballs. After the mouth is destroyed, the entire boss will explode and an entrance to the second base will appear.