This will be earned naturally in the space battle at the start of Act III. There are WAY more spaceships to shoot than 50. You should get it about halfway through the level. It is essentially a story-based achievement.
At the end of the battle you should also receive:
If you do not fire for the first 45-seconds of this level, you will net this achievement:
This achievement is dead simple on tadpole difficulty. You can take a good number of hits without dying. However, the simplest method is to hide in one of the corners of the screen. You are virtually unreachable there.
All in all, this level alone should net you three achievements in 5 minutes!