QUAKE 4

Complete the entire game on the corporal difficulty. The only thing to note about this achievement is that the difficulties do *NOT* stack, so to get all the "Completed the game with the rank of...." achievements, you must play through the entire game four times (once on Lieutenant, once on Corporal, once on General, and once on Private)

Here is a link to a playlist of every level completed on General difficulty. Simply follow what this guy does, and you should be able to get any of the difficulty achievements without much problem.

*NOTE*
Save often. It'll really make this game a less of a pain if you're not replaying the same few minutes of the game over and over.

http://www.youtube.com/view_play_list?p=4BFB72AC7795CECC

Not bad quality too, considering the game!

His run is about 4 hours, which actually doesn't sound so bad considering the ratios on these achievos.