You need to clear a mission without dying to get this achievement.
I recommend playing the first mission on Easy difficulty.
As soon as you land in the water, start running and constantly shooting. When you get to the building that spawn the crustacean monsters, through grenades until it explodes. Continue forward, killing the next to crustaceans and freeing the prisoner. He'll give you a Heavy Machine Gun.
Continue onward, killing a few more crustaceans, and stop when the big door opens. Stand here and do some "spray and pray": Fire the Heavy Machine Gun continuously whilst tapping the up button, killing every enemy in a 90 degree radius. The building will explode once the last crustacean is dead, and you can move on.
Next up you'll meet a heavily laden and clumsy soldier that'll give you Super Grenade. Don't go down to the submarine, but instead go up. Kill the crustaceans there, and just keep running and gunning. When the flying insect monsters appear, line up flying insects with the crustaceans so the flying insects are right above them, and fire the Super Grenade at the crustacean! The fire from the explosion will reach upwards and take out the flying insect monster as well!
Get to the bridge with the prisoner. He'll give you a Heavy Machine Gun if you free him. You can either finish off the rest of the flying insects with the Super Grenade + crustacean method, or take the Heavy Machine Gun for more spray and pray. You might run out of crustaceans before you kill all the flying insects, at which point you should just pick up the Heavy Machine Gun and spray and pray the remaining ones. Either way, make sure you have the Heavy Machine Gun when you move on.
Next you'll come to another choice of path. Go down into the boat with the prisoner. Get up on the bough(front) of the boat and start shooting straight ahead at the enemy boat. You'll kill a couple of enemy soldiers on the boat, and then destroy it. When exploding, jump onto the shipwreck to pick up more Heavy Machine Gun ammo(don't worry about the soldier, he's got his hands full), then back onto your boat. Fire 2-3 times at the sinking enemy boat, and you'll continue onward.
You can stay on the bough and just spray and pray backwards. When you get to a stop, throw a handful of grenades at the truck, and you'll move on. Do the same thing once more until you get past the second truck. Then go down to the deck and fire at the floating fish that's attacking.
Once done, you'll be at the boss fight. The boat will crash and explode a bit, revealing a tank. Jump into the tank and then jump with it onto the pier. Now the boss fight starts, as a giant crustacean monster with a military fortification on its back attacks!
The boss has two stages, while continusously destroying the pier, forcing you to move.
During the first stage, it will fire glowing cannon balls which you'll have to maneuver around. Stopping yourself from moving every now and then can help you avoid these easily. Your tank can take two hits from these, but the third hit will destroy your tank. So try to avoid this as much as possible. Just keep avoiding those cannon balls, and keep shooting. Don't worry about trying to hit it with your tank's cannon as the bombs arc downwards, but instead just use the main gun. Avoid picking up anything the prisoners give you, as it'll serve no purpose but to distract you.
After you've dealt enough damage, there will be a big, but quick explosion on the boss, signaling the start of the second stage. Once on the second stage of the boss fight, you should be safe: Just roll your tank up to the right. The boss will stop shooting the glowing cannon balls, and reveal a new, larger cannon that'll fire large bombs. These bombs will destroy the pier, but as long as you hold that right button in, you won't ever be hit. Just continue firing until the boss is dead.
Congratulations, you have now gotten the Ninja achievement!