LEFT 4 DEAD 2 : NEED ANSWER REALLY FAST !!!!!!!!?

I just bought Left 4 dead 2 for xbox 360. I'm already good at it and have a few achievements , But i want to learn how to be expert on the game and have all the achivemens and be able to play it on expert and own all of my friends on the game. How should i do this without cheat codes )