Either the Covenent or the Storm. In the Storm, enter the first open area and head for the building in the middle, Marines usually go there. On top there is a Missile Pod. Shoot the Banshees crciling above you.
In the Covenant, after u kill the first AA Wraith, a Pelican should drop with Warthogs. Grab your S-Laser and follow the beach straightforward, and look up. Two Banshees should be flying in circles., light em' up.
Still, I think the acheivment is glitched. I shotdown a Banshee with a Rocket LAuncher and still got it.
Oh, and for anybody having trouble with Two for One, do two of the following:
1.) get a mode such as Oddball or Crazy King. Wait for everyone to go near the Hill/Ball carrier, and form a tight group. Then fire a Splaser shot. Note: must be on lvl with a Splaser, preferrably Snowbound.
2.) Wait for two ppl to fight over a vechile ( a Ghost your best choice)start charging when you think the vechicles gonna get hijacked.(I.E. Some one going REALLY close to an active vechicle.) If you have good timing, and true aim, both driver and hijacker will die, nabbing you the acheivment.
I'm off to destroy Canada,they've had to good for to long...