How do you get the used car salesman achievement in Halo 3?

[JJAY]

I know how to "Get it" but for some reason even if I meet the criteria it does not give it to me. WHy is that and how do I fix it?

 

[Juke Box Hero]

You can shoot off the turret of a phantom I think. >Campaign

 

[Steel Chimera]

I got mine while in a scorpion tank going towards the Phrophet's Citadel (where you fight 2 scarabs at once). I shot an oncoming enemy vehicle with the tank cannon and got it.

Aim at targets at a distance... sometimes they will drive out and enemies will jump off when you get close to them.

 

[sean]

In campaign, go to Tasavo Highway and find a trip mine. Kill a chopper with that trip mine. Achievement Unlocked