After unlocking all other achievements on this game, and trying for 10 times or so, I finally managed to beat that scumbag Helios with a single life. There is no quick and dirty way to do it, just practice and a good knowledge of the four levels, but I will give you a few tips that made the difference for me.
1) The weapon
A Type looks cool, flamethrowers are always cool. But I did a little damage comparison, and its damage is pathetic. Forget it.
C Type is really powerful, but very hard to use. On top of this, you CANNOT kill the final boss with it (you cannot repeal the giant balls with it)
B type is the weapon to go. Plus its secondary attack is really usefull in the junkyard and against the final boss.
2) The shield
Tap the R Stick whenever a bullet comes at you (and begins to glow blue in your vicinity) to throw it back to your enemy. (Does not work on poison balls, however). Without it, turrets in tight corridors will hit you, the shield is the only way to protect you in such situations. You might even kill an opponent or two with it.
Most of the time you won't need to use bombs, and they will act as a secondary shield. However, there are a few situations where using them will save you a lot of trouble.
Third boss : when the weak spot gets on top of the screen and start attracting junk. Wait for the screen to fill, and release a bomb. Do not shoot the junk, concentrate on the boss, the bomb will get rid of everything onscreen. If you manage to damage the boss enough before that, you will kill him right away. No need to dodge those spining saws once again. :)
Final boss : when it starts to summon the meteors and the big balls whatever at the same time, (roughly 1/5th of its life), he will become vulnerable for a very short amout of time. Not enough time to do it any serious damage. Release a bomb or two, and good bye evil floating head :) Do not waste them before. Use the secondy shoot during the meteor shower, works pretty well.
During the final stage : any time you see the screen flooded by green beam and poison clouds. The bomb will protect you. Might even buy you enough time to get those 2 health recovery item at the end of the stage.
Final tip : there is a health recovery item "hidden" in the third stage, just in case. Just after you beat the first twin claw thingy, before the giant crane swings some piece of crap at you. It's located at the top of the screen. Use the acid thrower on secondary mode to destroy the garbage pouring from the sky till you see it. Saved my day many times.
Good luck. ;)