Truly for the masters, this takes absolute precision and requires memorization of the entire game. There is little to say but practice makes perfect. One thing I can tell you is that you start with 2 lives but can gain more from the more points you gain, which helps you stay on that 1 credit/continue. MUST be done in single play.
Helpful hints - Credited to RavenBlackThorn
From what I have noticed, every weapon appears to be upgradeable at least once.
The rocket launcher (first weapon pod) doubles its firing rate and increases its range if you grab the second pod by the helo.
The spreader (second weapon pod) increases it's rate of fire from 3 shots to 5 shots if you grab the second spreader by the stairs in level 2.
The grenade gun (third weapon pod) increases its splash radius from two fireballs that spread out to the sides to 4 fireballs that spread out in sort of a star pattern.
The laser (right before the end of the first level) seems to double its range if you grab the second pod by the first tank in level 2.
This added range is an absolute lifesaver in level 2. It allows you to out range all of the enemies and turrets. You can actually shoot all of them if you (and your partner *cough*) are patient enough. With the added range you can take out all of the enemies fairly easily as opposed to trying to run past them (which can be pretty risky).
In level 4, it makes taking down some of the tougher/stronger enemies a little easier as well (if you can make it through the sickness in level 3 without getting hit).