Also a good chance in Campaign on 'The Covenant'. Just after destroying the AA Wraith at the very start of the level, you'll have a Spartan Laser (if you've saved some ammo) when 2 Banshees come flying in.
I got mine on "The Covenant". Saved a Laser for when you take out your tower, but left it at the entrance. After I took out my tower, I flew over to the Arbiter's tower (the place where you can get the skull) and a few banshees were flying around, so I took one out easily. Takes a little more time I suppose, but if you need the skull too, it's pretty convienient.
If you do this on the Covenant, I suggest doing it on Normal or Easy. On Legendary, the Banshees will come in after you (I'm not sure about Heroic). It might take a couple of tries if you're not a good shot. Another point to make. On co-op, if you're not first player (the Chief) you won't get the Laser, you get a Beam Rifle.