North America (Hard) Achievement in Enemy Territory: Quake Wars

Just play through it as said. Bit of a pain since completing on Hard does not unlock the achievements for Easy and Medium, but ah well. For the class achievements, try and complete the objectives by yourself for the respective class (e.g. on most hacking anr repairing objectives, you can get to 2 star straight away by completing it yourself. For explosives, you gain 1 star per objective, and gain more xp by blowing up turrets etc). Also, on the maps you are defending for 20 mins, this is an ideal time to get the Medic/Techinician and Field Ops/Oprressor 3 Star Levels. For medic, just do lots of reviving, healing, and call in a supply crate near the object you are defending. This can easily be done on one map. For field ops, just do a lot of ammo supplying, but also use a Hammer Launcher and launch it as often as possible throughout the game at as many large groups of enemies as you can (this is a biggie since you see the bar jump up for big groups of kills). Position it out of harms way though so it is not getting constantly disabled.
For the 25/50/100 Stars Achievements (if you haven't already got them by playing though on Easy, Medium and Hard), playing on Africa as GDF on Easy difficulty seems to be the best bet (since you are attacking all 3 maps, making for easy Objectives/Stars).