The game have a 'anti-leeching' tool. If you delay a lot of minutes (between 4 and 5 minutes) on the same stage/screen, you will receive a hard 'rain of bombs', like that bombs the airplanes shoot on you. Try to do it on the stage 1-1, the easiest. And if you delay a lot of time in the same position of the 'enemy' bar, without progress, you will receive this rain of bombs easily. Yeah, if you are expert, you will can survive to these bombs, without die, using your skill on the 'rolling' move. You will receive a lot of items (bombs, weapons) if you destroy bombs or if you shoot at the bush.
General Tips :
1) Killing people and motorcycles (and rocks) won't get you many points -- don't bother unless they get in your way.
2) A well-placed grenade will destroy two choppers.
3) If you are in a corner and a chopper is firing at you, run farther into the corner. Even if you're already IN the corner, if it sees you trying to move toward the corner, you won't die.
4) If you find yourself trapped by incoming bullets, spin the trackball down and to one side (but mostly down). This will cause your character to 'duck and roll'. If you are rolling, you can't be hit.
5) Kill the snipers (the guys in gray bullet-proof suits). Half the time you'll get a grenade for killing him. If you don't, two medics will come running out to reclaim the body. Shoot these medics as much as possible! Each shot they will drop 1 grenade. But the limit is 4 grenades on the screen at the same time. Pick up these grenades as quickly as possible and shoot the medics again for more grenades. Sometimes they will drop new weapons too.
6) On the level one screen with the sliding doors : shoot out the doors immediately upon starting the level. This will stop the guys from lobbing grenades at you.
7) Airplanes are hard to kill. The best method is by using machine gun or bazooka gun on it. In the absence of a super-gun, you can lead a grenade ABOVE where it is on the screen. If you don't get it, and the three bombs are being dropped right on you, it IS possible to stand BETWEEN where two bombs are coming down. You WON'T die!
8) When there's nothing good to shoot on the screen, fire at a bush. This is the way to get extra gunnery! Sometimes you'll only get more grenades, but you'll eventually get a super-gun!
Tips For Specific Enemies :
1) The giant chopper that shows up at the end of screen 4 on level one doesn't give you any points until you destroy it. Shooting the orange bullets won't knock off any points toward killing the chopper. Only shoot the orange bullets if they're in your way. Otherwise, spend all your time shooting the chopper. Don't waste any grenades on the chopper--it doesn't do as much damage as your gun (and you have unlimited bullets).
2) The semi-truck that shows up at the end of screen 4 on level three will self-destruct if you destroy the CAB (the blue section in the front of it). On this screen you actually get the point for it as well. It is easiest to kill it with the machine gun and/or bazooka gun, but in the absence of either of these, throw grenades at the cab (leading the throw by about four inches). It is MUCH easier to destroy the cab rather than destroying all the attack sections that it drops off.
3) The three CPUs that show up at the end of screen 4 on level four can actually be destroyed with only 2 grenades (I had to do this once when my grenade supply got low), but the best strategy is to lob five grenades at one of the outside CPUs (note that you can only have four grenades per player in the air at any given time, so space them out a bit), then go for the CENTER one (with another five grenades). The final CPU can simply be shot with the standard rifle, and if you're standing on the opposite side of the screen from it, it won't be able to hit you. Don't waste any more grenades.
4) Once you learn how to defeat the final menace (at the end of screen 4 on level five -- the final level) you can get by it (rather) painlessly. Go all the way to one side. Use your gun to shoot out the two surrounding machines that are shooting at you. As soon as they are both gone, lob three grenades at the center machine. By this time, the two outer machines that you shot out have regenerated and are firing at you. Shoot them out again, followed by another volley of three grenades to the center machine. Repeat until the center machine is gone. This requires 20 to 40 grenades, so stock up on the last level (5-4). That is what that final level was for.