After my last tries with the game, I think it's good if I expand my previous tips, wave by wave.
If you want to score big...
Practice a bit surviving one mutant. It's going to happen. There is a relatively good way to deal with one, which is to trust a bit with some height difference from where the mutant is (be below it, for example), let it follow, reverse while sliding, and cover the height difference (if you were below it, go up) while shooting frantically. Because the mutant oscillates in height, it is very likely you will hit it. If you don't, run away until you get some more distance and try again.
1st wave: You should not lose lives nor smart bombs. You can lose humans, but only if your score is larger than 3k. You should not pass this wave with anything less than 3250, you should be able to get 4k regularly, 5k once in a while. I got 7k once but I had to use a bomb as things got out of control - I rescued humans but wasted a bomb early. I only got 6k once without losing lives and bombs, and that was when I got my best score - those extra 2 or 3k will matter a LOT when you're in wave 3 (or 4) trying to score enough to get an extra bomb or life, and clearly 3k is a relevant difference in high scores.
After wave 1: If you can rescue any humans instead of killing them, so much the better, but as I said before in wave 2 onward the most important is preventing mutants, so don't be particularly careful in aiming to the lander; if you waste too much time, things will get out of control.
Don't lose all your lives while you still have smart bombs, but use them well.
2nd wave: You should not lose lives. Personally I think 1 smart bomb is perfectly justified, don't feel too bad using up two bombs. If you can pass without using them and not losing lives congrats, you are in very good shape! But if you are using a bomb, use it well. My priority for the bomb is to destroy the annoying thing that bursts into more (motherships, I believe... There is only one in wave 2), but you should either get also 2 or 3 of those pink squares, or destroy 3 or so landers together with it.
3rd wave: If you got here without losing lives and only used one smart bomb, you are in good shape. Even if you used two, you will have 1 more by now to scoring (or 2 more?). As soon as you pass near the motherships (there are 3 in this level) try to catch all of them with a single bomb. Often you will nail 2 of them entirely and the 3rd just releases the tiny buggers. Sometimes I could flee and handle the tiny guys, but often the best thing is just to flee to kill some landers before things get too out of hand with mutants, and when the buggers catch up you probably have more stuff in the screen to justify using another bomb. Don't hesitate in using all your bombs here, although if you happen to get to wave 4 without bombs you're not lasting much, try to get to 30k to get an extra bomb. If you have 2 mutants and 1 lander in the screen, for example, that is a good blast in my book. If you have 2 landers with humans near the top, that is very good use of the bomb, as you can probably get 2k out of the bomb with the rescues, and you avoided two mutants.
4th wave: Good luck

I usually get here with some lives, but not many bombs (zero, one). If you can shoot enough stuff to get one extra bomb (you may have gotten to 30k just as the few survivers get accounted for at the end of wave 3) then use it on motherships, there should be so much chaos you will get someone else with the blast.
World goes BOOM: If they kill all the humans, which probably happens in wave 3 sometimes and as far as my skills allow, always in wave 4 always granted that you survive that long (I never got past wave 4 myself, although usually they kill me before they kill all humans), DON'T PANIC. It's actually surprisingly survivable (surprising as in, more than you thought, not as in very survivable). You can squeeze some extra points if you have 2 lives left or so (for my best score I got some heavy points after the world went boom). I recommend not changing directions often to start moving in the other direction - although you should definitively reverse to fire back at mutants chasing you (see a good way to dispose of them above). You should never stand still, and moving too slow is not good either (which is why changing direction is bad - you will need to stop to start going in the other direction).