This is an extremely easy level until the very end. Just keep running right and killing enemies. Occasionally gallimimus will run at you. Just jump over them. You will not have to worry about any other dinosaurs, and the gallimimus only appear two or three times in very short bursts. You'll also have to deal with no annoying flamethrowers, just gunmen and grenadiers.
You can keep the machine gun equipped for the entire level if you like, picking up additional ammo for the machine gun. Halfway through, a helicopter will fly overhead. Stay at the far left edge of the screen. It will drop five bombs across the screen. Jump to avoid splash damage just before the impact of the last bomb, then continue as you were before.
At the end you will see a medkit followed by two ammo pickups. Switch to the shotgun and pick them all up, because you're about to face the helicopter, which is the hard part. This time it will have a big cage attached to it and you must remove it first by shooting it. Duck under it when it gets too close, but keep shooting when it passes. Soon it should drop. If you're fast enough, you'll get it off just before the helicopter disappears, after which it'll come back with more bombs to drop on you. This is very difficult to dodge when the cage is still attached. If the cage is off, do not stand on it; wait around the left edge of the screen. This time there will be more than five bombs and you will have to jump through a gap in them. It's difficult, but you can do it. Then jump on the cage and quickly and continuously fire at the helicopter. It will fire a few bursts of bullets at you; this can cause serious damage if they all hit, so jump over them, but don't stop firing! The helicopter will not take very long to defeat with shotgun shells, but be sure you defeat it before it defeats you.