I did as many missions as I could on Veteren and then the others on the lower difficulty of: Hardened, to obtain this achievment.
Type in RoosterTeeth or NextGenWalkthroughs for Call of Duty 4 Veteren guides on Youtube. I used these and are very useful.
My own tips and things that I learnt through completing the campaign on Veteren/Hardened:
-Stuns are a man's best friend, always use them.
-Light Machine Guns (LMGs) like the Saw and Rpd are the best weapons to pick up as they shred through enemies and there always comes that annoying moment when you have to reload.
-Grenades usually get me killed more than the enemies shooting at me. Frags have an absolutely huge radius in which they explode. Stay clear of them and always throw them back as they are not cooked in single player.
-The checkpoint system is crazy, you have to take out a hordes of enemies before you get a checkpoint sometimes. It seems like they were just randomely placed.
-Always be cautious going around corners. Staying prone or crouched is a good idea.
-The AI have some amazing aim sometimes with their grenade launchers and Ak47s etc.
-Also the AI can flank you often so use such things as claymores.
-Let your teammates go first and listen to what you say, so you can see where they are shooting etc and also they have infinate health.
-If you don't keep pushing forward you won't reach checkpoints as the enemies infinately respawn.
-Don't rush in on Veteren, you'll get nowhere.
-Take the strain off and complete it on Hardened. Was so much easier for me.