0 day Attack on Earth

This achievement is for finishing the entire game on normal or hard difficulty without dying once.

Which means you must finish ALL 21 stages without dying once. Fun, yeah? There are five general types of enemies (named by the color of their blip on the radar :

Orange dot enemies that you need to kill to progress
Yellow dot enemies. These are special types (you get achievements for killing a certain amount) and typically have fairly powerful attacks
Green dot enemies (big) they have a good amount of health and some decent attacks
Green dot enemies (small) these usually spawn from the clouds, take one hit to kill, and may/may not have projectile attacks
Green dot enemies (spawned) these are enemies that are spawned from bigger enemies

Some general tips :

-The radar is the difference between who loses and who wins. It has a bigger radius then your screen, so it's an early warning system and unless you like redoing stages over and over again, learn to use it.

-Each type of aircraft has a different weapon. They all start out with a little machine gun, but as you get more of the green health orbs, it upgrades to something different :

Fighters gets 2 and then four missles that shoot out every couple of seconds, and they keep their machine gun. This is really good for smaller enemies, however the damage against bigger ones is small. Unless you get all four missles to hit, it does less damage then other types. The missles like to track enemy projectiles that can't be destroyed, further decreasing it's usefulness.

Helicopters shoot flames, and once you get enough orbs, it shoots both in front and behind you. The damage is decent, and the flame spread that shoots out the back can be nice, as well.

Reciprocating's machine gun turns into 3 and then 5 seperate shots. Again, works well against lots of smaller enemies, however against big enemies and bosses it is especially weak.

X-A gets a missle in place of the machine gun. It does really good damage, and is especially good for bosses. It shoots through armor, like the New York boss. My weapon of choice, same one I used for this achievement. Sucks against small enemies though

-Your allies are good for two things : meatshields, and using their guns on small enemies. Otherwise useless.

-Your nuke is more or less the panic button, use it when you are surrounded. You can also use a nuke to get the mosquito type enemy off you. The ally AI is basically useless for this, so keeping a nuke or two for mosquitoes is suggested.

With that, area by area :

New York

Orange : Octopus-like things. They have 8 tentacles that will take away two bars of health, avoid at all costs. If it pulls it's tentacles in it is about to shoot flame out of it's (mouth?) and then swing it's tentacles around. Get far away. If you stay away from the tentacles, he is fairly easy. In the last half of New York, the front two tentacles get closer to it's head, making the safe spot smaller. Beware.

Green : Mech-like things are the big enemies here. They shoot out some kind of missile that explodes into smaller enemies, these will make black clouds. Easy to kill.

Another green enemy is a type of pod...thing. It will release 5 UFO/ship/somethings. The UFOs have no missiles, and are easily dispatched.

Boss : a fortress type enemy with a bunch of walls that spin around it. It has a volcano eruption type of attack, which spills balls of death at you. When the shield walls drop, it will start shooting out projectiles in a circular manner. If you have the X-A you can just go crazy with missiles, otherwise you will have to shoot when you have an opening or the wall is down.

Tokyo

Orange : Big ships, with two forms. The first form has a core surrounded by barriers. You'll need to destroy all three so you can shoot inside. The X-A's missile very easily destroys the armor, and pierces through it as well. It will shoot out blue projectiles in a circular pattern out of the blue spots on the ship. When it stops, move in and blast it. Repeat. Once it dies, it explodes, flips over, and reveals it's (real?) core. When it's flipping it can damage you so back off. The second form has the same blue projectiles, however flames also come out of the four cannon type things sticking out of it. Again, wait till it stops, then move in and smash it.

Green : The main green types are mech type things with four legs that shoot 5 lasers out of it's cannon. The middle laser moves faster. Get behind it and blast away. I do not think the legs can damage you.

The second type is some kind of machine core with 4 "arms" around it. It can extend the arms to try and damage you. Does not actively try to engage you, so you can kill it fairly easily.

The small green enemies in this stage are jet fighters. They move fairly fast but have no projectile attack. Easy kill.

Boss : 3 spider mech type things. 4 legs. It's body moves up and down, when it is down, you can kill it. When it jumps in the air, RUN FAR AWAY. If it falls on you IT.IS.INSTANT.DEATH. Once it lands (you will hear a noise and see a shockwave) move in and attack it. While it is flying, and sometimes when it is on the ground, it will release mines. Destroy these, as they will be a pain if they stack up. It may also lower it's body and spam lasers everywhere, however if you stay in front of it's core/face, you will not get hit, and you can smash it.

Paris

The orange dot enemies here are animal type things. It's only method of attack is shooting out these orange/yellow projectiles that explode and shoot out smaller projectiles. It's head is the weak spot, so stay in front of it and shoot it, moving away when the projectiles show up.

Green dot enemies are where it gets crazy. The first type is a snake that pops out of the ground and shoots a bunch of projectiles in front of it. Get to it's side/back and shoot it. If it reburrows then watch the radar to see where it moves and chase it down.

The other big green type is probably the biggest pain in the game. It shoots projectiles out and moves at a decent pace. Once you kill it, it spawns 10-20 little maggots. They move, but have no projectile attack...until you kill them. When you do, they shoot out three bullets. EACH ONE WILL SHOOT BULLETS. Be careful.

The little enemies are ships that fly at you and can shoot projectiles. You have to stay on your toes, this stage is the hardest in my opinion.

Boss : The boss is a giant snake. It is so very easy. When you hear a noise and see a little flame animation appear on it's body it is about to shoot flames. Dodge it. The only other attack it has is trying to hit you with it's body. Smaller green enemies spawn, and yellow ones too. Watch the radar, and you will finish this stage easy.

Yellow Type Enemies

Mosquito : These orange balls will hunt you down and try to latch onto your ship. If it does, it will slowly drain health until someone kills it. You can't shoot when it's on you, so your only option is to hope an ally kills it, or nuke. Always nuke it while you are going for this achievement. You can't take chances with ally AI, it is dumb.

Sandwich : Two machines that will try to fly above you, and when they are above you, they will smash together, smashing you if you are in their path. Once they start to move down, move out of the way and shoot them. They will move away, just repeat. Fairly easy.

Worm : Long worm enemy that moves fairly quick. No projectile attack, so keep your distance and shoot it. Easy.

And that's that. Good luck!