In the level where you spawn with a Spartan Laser (The Covenant), move forward until you destroy the Anti-Air Wraith. Save a few shots in your Splaser and just past where the Pelican drops off the Mongoose and Warthog, there will be a group of Banshees circling above. Keep moving towards them until one or two starts heading straight for you. You should have an even target and be able to take one down on the first try.
On the lower difficulties, don't even worry about dodging the Banshee's fire as you can withstand it. Just concentrate on hitting your target.