I just got the achievement for completing all 128 levels and I figured I'd share a few things I picked up during the process that really helped me out, and that may help anyone who, like me, decided they're gonna have a go at completion:
1. A big one this, and something my dumb ass didn't realise until I got to around level 100 but helped me a LOT. Sometimes I noticed that weapon power ups and bombs were coming down full throttle and majorly helping me out and other times I couldn't figure out why I wasn't getting any help. Then I realised, and it's really obvious now; there is only ever one item, aside from fuel, that is allowed on screen at one time. The second you have collected either a weapon upgrade, bomb, gold guy or orange triforce thingy, a new item drops instantly. So even though most of the time new weapons and bombs are what you're after, going out of your way to get an orange thing will always trigger a new random item to fall. Staying on the items as well as fuel will always mean you have a constant inflow of weapon power-ups, bombs and points.
2. Sort of contradictory to what I last said, but on the later levels especially, a three-way shot is more often than not much better than a more powerful horizontal shot. On the last levels there are so many enemies it's absolutely mental and a three-way shot is your best bet for clearing the way and even staying alive.. However, weapon upgrades transfer from one stage to the next only in the regards to the weapon's power, not the weapon itself. Weapon power moves up in groups of three; horizontal, horizontal-vertical and three-shot, AND THEN, more powerful horizontal, horizontal-vertical, three-shot, AND THEN the same again more powerful etc.
If you have the first-level three-shot weapon and finish, you still start the next level with the first-level horizontal version. But, if you get onto the next power level, you begin with that superior power level again.. until you die and start again.
Even if you've been avoiding the weapon upgrade onto the next level because, although it's more powerful, it's not as useful and you won't survive for long without the three-shot (mainly on the harder levels), collecting the power-up just before you finish the level will mean that you start the next level with a souped up weapon.. Although it's not worth dying for, obviously.
3. Another one I didn't figure out till much later. Although with the horizontal and horizontal-vertical shot you can't change your aim, with the three-shot you can. Really, try it. If you press up or down, Jetpac guy actually moves his arm to shoot diagonally meaning you can cover even more reach more accurately. When you're moving up to collect things or moving down to your rocket, or whatever, this means you can really really clear the way.