Originally I asked about how to get the 'Sniper' achievement, since I got it, i'm just going to use this thread to try and detail each achievemnt I run across since there's no decent guides.

Achievement Name { Game Title }

Sniper { Yars' Revenge }:
You have to shoot the cannon (not your basic guns) at the center disk when it's spinning but it only counts once it releases and flies at you. This makes it much harder. To get the cannon, you have to clear enough of the red bricks away and go physically touch the crosshair looking thing (just not when it spins).

Exterminator { Millipede }:
On the first stage, I had to destroy a millipede with the initial bombs that were supplied, once that was done, the stage shifted and I saw the bombs were replaced by ones with DDT written on them. I destroyed that and it popped. It could just be different colors and I couldn't see DDT written on the initial bombs, but doesn't explain why the achievement didn't pop until the 2nd stage. Let me know if wrong.