Banshee fun at the start
I've seen at least a few walkthroughs of this level which say that after exiting the lifeboat, you should immediately cross the bridge and follow the cliff until you get to some large rocks where you can hide until the Banshees leave. It's advice like this that makes me think I'm playing on another wavelength entirely. Why on earth (or on Halo) would you want to miss out on an entire section of fun? I suppose you might if you just want to 'beat the level' with minimum effort, but that seems pretty shallow to me. Personally, I play to have an optimally enjoyable experience. I want everything the game can throw at me. So check out these Banshee ideas.
Armed with your pistol, the Banshees are no real threat if you're a decent shot. In fact, as long as you're a good mover, they're not even a threat if you limit yourself to just an assault rifle. One thing I always want to do is bring them both down. I mean, it feels so good to see those Elites go flying on without their machines around them. Wonderful stuff! But once you bring down one, the other starts flying off. So, the normal tactic would be to weaken them both to near destruction before bringing either down. If you want to get hold of a fallen plasma rifle, take care to bring it down in a convenient spot.
Here are some Banshee challenges:
(1) Quick-fire double destruction
After leaving the lifeboat, cross the bridge and go left, behind the large rock. One of the Banshees will dive at you. Destroy it, and then use rapid and accurate pistol shooting to destroy the second before it flies off. You should just about have enough time. As a sub-challenge, see if you can get that first Banshee on its first swoop, using only the assault rifle.
(2) Get a plasma rifle
Bring one down where you can then use the plasma rifle (and possibly plasma grenades) to dispatch the landed search party.
(3) Collateral damage
Bring one down at just the right time so it crashes among the enemy, killing some of them. Not easy!
(4) Tag the Banshee
After dispatching the search party in order to get hold of plasma grenades, take down a Banshee by tagging it with a plasma grenade, simultaneously taking care not to be severely killed. Then use the pistol to bring down the second before it flies off.