Halo 3 - Too Close To The Sun Achievement

Destroy an enemy banshee with the spartan laser or missile pod in a ranked playlist or in campaign

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This is easiest done in Campaign, considering Matchmaking isn't very populated anymore. On the campaign level: The Covenant, you spawn with a spartan laser, all over the sky there are Banshees flying around, simply shoot one of them down. If you run out of ammo before you get one simply reload the checkpoint and try again.