Important to know for all bosses is that each of them comes in three different versions: Mega, Giga and Tera. The Tera boss is the hardest and will only appear if you manage over 98% shot-down in an area first. The Giga boss will appear if you get at least 95% shot-downs and you'll get the weakest of them, Mega, if you get any shot-down rate below that. The version of the boss will be displayed in the "console" window just before the boss, in case you're curious. This is also helpful if you try getting at least 95% shot-downs everywhere - if you should only get a "Mega" version, you don't have to do the boss battle to see that you failed that goal but can restart the level immediately.
Area 1 boss:
This one is rather easy to finish. At the first stage, you basically have to shoot the walls around it down and then you can hit it. From time to time rockets will shoot at you; these are somewhat special as only the middle one can be hit at first, it creates a shield around all the others. Once you shot the first one down, you can shoot all others down too. If you are going for a high score, you should be patient at this stage of the fight and not shoot anything directly *at* the boss - instead, just try shooting down as many rockets as you can. Still keep in mind that after some time, the boss will automatically move on to the next stage; it would be a shame to have not shot down some wall parts at that point.
In the next stage the boss will move it "tentacles" around until two of them start glowing in green (he's charging a rocket there) - if you are a bit fast you should be able to keep him from shooting anything there by just shooting those green spots down quickly. The boss will then build up another wall around it, see above on what to do.
At times the boss might also decide to turn around, shooting rockets outwards in all directions; the rockets usually won't hit you though, so just keep shooting at the glowing green end of the tentacles. After this there is another wall built up, and also another tentacle attack just like before. If you are going for a high score, don't shoot at the wall here at all but only concentrate on the rockets; otherwise the boss will already move on the the next stage (hitting around 20 of the firewall plates is already enough for that!) and you will effectively lose rockets to shoot at. Be patient, take your time.
This last stage of this boss has it materialize a ring of blue boxes turning around it; over time, little spiky disks will evolve out of these boxes, flying right at you. If you just want to take down the boss, you can focus on it, just defending yourself with rapid fire against any oncoming spiky discs. If you want a nice score, take your time here and just shoot down the ring and the discs as often as you can, never even once shooting at the boss. Once you shot down all the blue boxes, a new ring of them will appear so you can rack up quite some points this way until the boss is destroyed after four or five times of spawning those enemies.
Area 2 boss:
This guy tries hiding from you in some sort of fan-shaped structure. At first, the boss will fire shots at you from one of its "arms" - at first those shots will miss you, but if you wait too long the guns will sooner or later fire at you, so take them out fast. After that, the fan will close around you and then try to mash you with walls that are coming at you. Just shoot the four small yellow points that stick out of the walls to destroy them; at the end there will also be a few big yellow points that are exactly in the middle, you have to hit each of those 8 times before the wall is destroyed.
Remember the very first stage of this boss fight? Well, the next one is exactly like that. Next thing, you will see the actual boss come out of its hiding place. So what are you waiting for? Shoot it as much as you can! It's not possible to kill it at this point though, because it will hide again and then the next stage begins. Some sort of tree structure will be created and small yellow spots on the twigs will spawn yellow jellyfish enemies that you shouldn't let get to close to you. You also have to shoot down the yellow spots; the tree will rebuild at some time so then you'll have to do it all again, but luckily afterwards it will be finished.
Now it's time for another first stage. After that the boss will appear again, and if you keep firing at it enough now it should be a piece of cake. If you are going for a high score, don't shoot too much at the boss at this last stage. Instead you should shoot at the bombs that it spawns: nicely enough it's always 8 of them, perfect to rack up some points here. After some time the boss will self-destruct anyway.
Area 3 boss:
Here the boss fight will not present the actual boss to you; instead, there will be many stages to actually get to it. The first one gives you the opportunity to score quite some points already: you will be flying to the boss arena, and boxes will kind of jump around. Shoot them, if you destroy some of them you can get a few bonus nodes here, giving you valuable points if you're going for a high score. Only after that the boss will actually initialize. In the first stage you'll fight a wall of lasers; jsut shoot them down, a good way to keep getting lots of targets at once is by just moving your crosshair up and down at the rightmost border of the whole wall on the screen. Be careful, aside from the common blue lasers that just shoot a beam out horizontally there are red laser that will shoot diagonally, making it hard to spot if you'll get hit by them on your way or not.
You enter the next stage when the protective wall is taken down completely, and now you'll have to destroy lots of what I'll call "batteries of enemies" - those grey things with a red line on them. You should focus your fire on those whenever possible - they give you lots of points, and if you manage to ake all down before time runs out and you move on to the next stage of the fight automatically, you'll score an extra 5.000 points for this. Most definitely worth it. But it won't be that easy, because there's a reason why I called those things "batteries of enemies" before - out of each of them, when you fly by there will be a hell lot of enemies spawning! You only have to be afraid of the red and purple ones, the other usually won't ever reach you. It's nice to earn some points to take those down; however as it's more important to take the batteries out because otherwise the enemies won't stop coming and will just keep blocking your whole field of view, you shouldn't focus too much on them. However you should use any overdrives that you have left at this point to take one enemy swarm out, as this will give you a great opportunity to shoot at the batteries before new enemies spawn again.
In the final stage you won't need any overdrives really. Instead, you're going for the actual boss now. It's still hidden inside a thick tube that is turning around all the time. On the upper part of the tube are red spots. It helps to also have your crosshair just on the rightmost part of the tube on the screen, because it's turning by itself you will quickly gather a fully charged shot. If you manage to shoot down all the red spots, the tube will open, giving you free space to shoot at it with lots and lots of charged shots. The tube will close again after a short time and the whole red-dot-shooting is repeated. In case you disregarded my earlier advice and still have overdrives left, feel free to use them now - it will take away around 80% of health at once! Your enemies here are red rockets that actually spawn out of any red dots that were not shot yet. You can mostly take your time before you need to shoot them down, as they'll travel a few times around the whole tube before they can even hit you. In any case, make sure that you shot most of them down before you shoot down the last red dot, otherwise they'll go on your nerves when you're trying to only hit the boss inside. On the other hand, forget about taking all rockets down and only then focussing on the dots - when all rockets are down and there are still red dots left, rockets will respawn from those! This is also the tip I can give anyone who wants to collect points: take your time here, be patient as usual, and just try to only take the rockets down.