A City in Trouble
After pressing start, you'll be thrown onto a highway. As you make your way right, you'll notice cars driving past - some of them in pretty bad condition. Immediately after, you'll face your first enemy: a rolling spiked wheel.
After the wheel, there's a missile guy. Jump the gap, then get ready to face several flying enemies that carry spikes underneath. When you destroy them, they drop the spikes. If the spikes land on a part of the highway that is vulnerable, it will destroy it. You'll notice that if enough of the highway breaks away, a gap can be created. Also notice that the only breakable parts are between the poles that support the highway. The sprites for those sections (they are square and sometimes have a street lamp on them) are invulnerable so if you time your kills correctly you won't have to worry about falling or jumping around uneven terrain.
Sub-Bosses: Bee Helicopters
Just continue, making sure to kill everything ahead of you. You'll next face off against a giant bee; simply run to the left side, then short hop and fire until it dies. The highway will cave in under you, so you'll have to wall climb the right side to get back up. You can also pick up some energy capsules by jumping down in the next pit.
Continue on and you'll face some walkers (they only take two shots, so just make sure you hop to get them quickly) and another giant bee. This time, the bee will spawn walkers so take it out before they can reach you.
Crumbling Blocks
As you walk in this next section, parts of the highway will start smoking and then fall out from under you. They are easy to recognize, often in the form of a single piece of highway hanging off the edge. They are also a different color. One funny thing about them is that you can wall climb them without them crumbling.
You'll come across two more types of flying enemies. The first is a wasp like robot that just flies horizontally, the other is purple and similar in appearance to the robots with the spikes underneath. These purple enemies are slow, but will follow you around. They don't shoot anything so take the time to blast them while they appear. The way they are programmed is such that when they appear on screen they fly towards a designated spot before going to attack you. Also, when they reach that spot they just sit there for a second before taking action.
Fun with Vehicles
The last enemy you'll come across is a robot driving a car. Just fire quickly and you'll blow off the robot, even though the car is still driving. You can jump onto the car and ride it, but make sure it's going in the right direction! The cars can only turn if there is a robot driving them (jump over one, wait for it to turn, kill the robot and then ride it wherever you want).
When you come to the long flat stretch with lots of cars, you'll see a massive ship floating above dropping them from a loading dock that lowers slightly. Run to the far right edge where the screen ends, and just take out cars while you wait. After four or five, the boss will come after you.
Boss: Vile in Mech Armor
This is a storyline battle, so you can't "win" (Vile is invincible). Once your energy meter drops below a certain point (at around four or five left), Vile will start shooting "energy nets" (glowing yellow orbs) at you, making you unable to move. Once you're hit by one, the story will proceed and the level will end. Keep in mind that if you're hit enough times before being trapped by the "net" you can still die; so try and get hit.