On The Ark, go to Rally Point Alpha and you should be at the tank part.

Go to the door which the Monitor is going to open.

Before you go in, get out of the tank and you will see a Grunt driving a Ghost.

Shoot the Grunt and take the Ghost. Reach the part where you destroy two choppers with one Ghost (there will be a destroyed Pelican too).

Keep walking until you see another destroyed Pelican, and a rocket should be at there too.

In front of the Pelican, you will see a Warthog that is destroyed.

Get on the turret and keep shooting for fun if you like.

There you go! A Warthog is destroyed but the turret is still working.