This achievement was rather easy. Take out three of the four banshees found on the level halo, not too bad right? Well I can guarantee everyone out there can get this, you just need a little determination.
Setup= Halo Anniversary campaign, the level Halo (level 2), SINGLE PLAYER (team-mate banshee kills count separate from yours...), you can choose a difficulty, it doesn't matter that much. I did this first try on Heroic, but if you're having difficulties, feel free to try Easy or Normal.
What to do= When you start the level you're leaving your drop pod that violently crashed and killed the crew. Grab your ammo, and head straight across the bridge. A search party will then come out (consisting of: 1 phantom [drops ground troops], 2 banshees)
-Hide behind the first couple of trees, and wait till both banshees get a little closer.
-When they are close, take out your pistol and scope in on each of them, taking a few shots at each. This will draw attention bringing them closer for an attack.
-While they are attacking, when one gets into a close enough range, unload on it with the assault rifle until it's taken a decent amount of bullets, and the vehicle is smoking.
-Repeat this process on the OTHER banshee till it is smoking as well. After the second banshee is smoking, reload.
-Make sure both are in a decent range, and use your assault rifle to shoot one down, then the other *NOTE once one is shot down, the other will try to escape. You then quickly shoot it down, and if it moves out of range switch to pistol.
-IF IT GETS AWAY RELOAD CHECKPOINT and try again.
After those two you'll have a ways till the next. You'll be driving in your warthog in a forested area looking for survivors when the other 2 attack.
-Jump out and get in the turret on the back, and use the same process as the first two (except with a turret).
After the first one, your achievement should pop, unless you only got one of the first two, in which case you'll have to take down the second one. Some might prefer to kill one of the first 2, and both of the second group. It still works.
****Please leave comments of how I can improve this post, and give a positive review. If negative, leave a comment why so I can fix it.****
Update-
Note worthy from "Hullabalooza"- One play-through does not mean completing the level in one sitting. If you stop and save and come back to the level later it will keep record of the banshees you have already killed so that you can still earn your achievement.