I didn't like the campaign much, it was pretty easy even on legendary alone I play it for online, and well, I'm good at it, which is why I do lol
i did play halo 2 online for a while, i got half decent. i had a level of 28 or something i think. but then i had to get live cancelled because my parents got rid of their credit card.
I don't know how much of this is actually teachable It's basically knowing what to do in certain situations, and aiming for the right spot in the head, and strafing back and forth so you are harder to hit