Metal Slug 3

First of all kill the enemies at the end making sure that one of them turns you into a Zombie. You wont lose a life but your character will be much slower and react slower when you need him/her to jump. Instead of grenades your power up is now a line of zombie puke that sweeps across the screen from bottom to top depending which way you're facing.

This is not that hard to do, the only awkward part can be trying to avoid the 'human' potion that appears as upto three of them can appear. Once clear of enemies and the boss kicks in, you will have several aliens flying in a circle above you. They are pretty big so its hard to miss them, but the best thing to do is to quickly use your button puke attack which will wipe out the aliens pretty quickly. Try not to get yourself stuck in corner, keep your movements consistent from side to side to avoid any possible shots the aliens will get off if you dont kill them quick enough.

Then the next part you just need to destroy the glowing egg type thing that appears while avoiding the 'headstones' that try to land on top of you. Again if you are quick enough you can just use your attack to quickly destroy the egg and there you have it, Achievement unlocked!

Here's a quick video showing how its done (Credit to BLK0314)

Change the Gamesetting on the EASYest level and you will get the Achievements anyway ;-)

When you reach the Finall Boss go into the Left Corner and fire imediatly when they start to rotate! Maybe one will survive the attack so shot once more and you should get them all down during seconds!
I remember the last one of them him me same time at the end and we both died. But i got that achievement anyway!

And remember to change Difficult on EASY and LIVES AT Beginning to maximum (7 i mean)! It works on all other Achievemets too...
Kind Regards