Part 1
The first batch of enemies approach from the top. Gradually move down as you defeat them, and collect the speed powerup. There will also be a missile powerup above, which you need to collect as well.
When the larger enemies approach, watch for a 3-way spread near the top of the screen. While the spread may make it easier to go through the tight tunnel, you could keep your existing weapon to fight against the boss.
The boss of the stage is a fast moving enemy, but attacks in a fixed pattern. It starts by charging forward, and releasing a series of bullets when it retreats. While it's advance and retreat provide a perfect opportunity to damage it, your position must be precise to avoid being killed.
 Part 2
The first powerup at the bottom is a protection, and the second is a wide hoop projectile. When the corridor becomes small, you can collect two speed powerups if desired.
At the end of the stage, you will need to shoot an eye. With the weapon powerup, you should be able to park on the same level as the eye. The weapon is wide enough to hit, with the bullet just out of range. In addition, the two cells that are launched from the boss are easily defeated by your continuous fire as well.