Complete the Ghana mission on any difficulty. The only parts that might give you any trouble are the big firefight in the room with the waterwheels and the end boss. For the firefight, use the assault rifle and keep moving, using the crates for cover. For the end boss, circle strafe him to lower his heath (he probably won't hit you at all if you do this), use Y to target the rock covering the metal bits under the ledges and then use X to pull the metal bits out and destroy the shelves. After the shelves are all destroyed you can kill him off by circle strafing. I recommend doing the entire game on the hardest difficulty as the game is not all that hard - difficulty only affects the combat (as far as I can tell) and there are only a few combat encounters per level.