Ok, so this is a walkthrough for this game.
Well, it's a small guide with some tips more than a walkthrough..anyway, let's start.
You kill your enemies by clicking on them, and then clicking something else.
The enemy will be attracted to that something else and vice versa, unless that something else is your planet or a spike.
Anything caught in between the two things will get crushed when the collision occurs.
Also, if you select two objects close to each other they will travel together when attracted to another object.
So, your enemies are essentially these:
Various forms of fast enemies (some of them have more health than others-pinks, greens and oranges have the most health)
Medium-Sized enemies
Large enemies
There are also some special forms of enemies..these are the stealth ship, that can turn invisible (you'll still be able to select it though), the whirly thing (it has two forms, and only one of them is selectable), the rocket launching ship (when it launches its rocket send it back to it) and the doomsday device launcher, that launches a timebomb..if that timebomb isn't neutralised in time (lololol) then it will spawn loads of small missiles.

The game has the usual "wave" system, and sometimes at the end of a wave you will get health orbs, maybe a nuke orb (a nuke destroys all non-boss enemies on screen, but they won't give you health and any orbs on screen will de destroyed) or a spike orb (a spike orb grows a spike on your planet..make enemies hit it instead of you as a last resort).

So, on to general tips:
Never attack enemies that are too far away unless you're sure that they won't touch your planet...you don't want two massive ships collapsing on your planet because of a miscalculation...
Try to kill fast enemies when they're close to each other because if you select them from afar they'll act wierd.
If you can, try to pair enemies that are identical or have ideantical healthso that they neutralise each other as soon as they crash.Usually here size matters (bigger=more health).
If you have lots of missiles coming your way, try to make them hit each other as quicky as you can.
Doomsday devices are always a priority, make sure they don't explode.

The first boss is tricker than one might think...he launches 6-7 rockets every now and then, and you have to deflect them all in order to survive (your planet won't be very tough at that time)...just select them and send them back quickly.Nothing more to say.
The second one has a shield that makes him unselectable, however he can spawn normal rockets, small ships and a massive rocket.
Just attract the projectiles to each other when they're near him and he'll be caught in the blast.
The final boss is more of an endurance challenge..he can make himself small and teleport...
He spawns lots of dommsday devices...if he's small, he'll spawn small ones and viceversa.
When he's small he spawns lots of them, and he's much faster, so it's harder to hit him...when he's big it's a piece of cake.
If he teleports while a device is heading towards him he can save himself, so some of your hits will miss.
And make sure that you always have him selected so that you can immediately select the devices and send them back to him when the opportunity arises...

OK, so now we go medal-specific:
Straggler: when the final boss explodes, let one of his missles (if any) survive...if there are more than one, make them hit each other.
Hiroshima: activate the nuke when you are being swarmed by enemies...
You're a winner: win
Al Gore would be proud: minimise the damage to your planet and get all health orbs.
C-C-C-C-Combo breaker!: this one's a bit tricky.you can either get it by trying to (but you'll have to dedicate an entire playthrough to get it), or play the game and get it by chance (I got it on my first playthrough).
C-C-C-Combo!: get a massive multikill...destroy enemies when there are loads of them close to each other.
Double Whammy: you'll get it by playing...
Humanitarian: select a health orb and give it to an enemy..preferrably a weak one.
Sidekick: make sure a spike survives the final boss
USA!USA!: use 2 nukes and then finish the game with one left.
Use the force!: you'll get it by playing
You're a real winner: well, don't die :D
Pentaforce: get all 5 spikes on your planet without any of them being destroyed.
That's no Moon+My Ears!: dedicate a playtrough to do this (unless you want to continue playing after you died).when you get your first spike orb, don't take it.when you make it to the boss, send the spike to him and then attract him to you.

Ok, that's it, have fun, earn medals!