its simple you just have to memorize where the snipe shots came form, I mean Halo 2's AI was so crappy that they didn't adapt to your death unlike Halo 3 and Reach and ODST do, so just memorize where they spawn and it's easy picking.

Elites just noob combo quick br swipe, duck n hide, then repeat. grunts just swipe the br, jackles same thing or snipe them. or grenade hell hunters just dodge and destroy them,

only challenging enemies for me were the buggers...