While all the game is technically an auto-scroller in a sense. This level scrolls on it's own as well. You will be in a vehicle the whole time. The level enemies are not too difficult compared to Level 2. The real difficulty here can be the level scrolling. There are barriers in the level that will hurt you in the roadway. I'd recommend trying to think in rows rather than the whole space of the level. Limit your vertical movements for when you need to dodge the barriers coming from the right side.
There are only a couple of waves to deal with here before the boss. The boss here is going to need a little detail:
There are two waves/sections. Both sections have a bit of "bullet hell" to them. The first portion your movement is much more limited and enemies will spawn onto the screen. Try to manage them while dealing damage to the boss.
The first section ends when the left half of the boss is blown off. At this point, more bullets will be fired as well as the large blue shots. The enemy spawn will stop however. While using the stopwatch can make the shots easy to dodge, it is either a glitch or feature of the game that the blue shots will redirect to your new position after the stopwatch duration ends.
Depending on your upgrades a use of one stopwatch or dynamite when the boss is at 25% health might be more than enough to get through safely (or none at all if you're comfortable with the dodging).