halo 3 and cod4

ok im trying to beat halo 3 on legendary and cod4 on veteran ive gotten to the second level on halo and like the third on cod4 can anyone give me any tips on how to get farther, better, or to beat both of the games without cheats id really apreciate it :D

Halo 3 - Four players online coop helps, a lot.

Call of Duty 4 - Persistency is key, and don't forget about the invisible war line that call of duty is famous for, the enemies will not stop coming until you move past a certain point and your teammates follow.