Warthog on the Shade plateau
When you get the Warthog, it's possible to drive it up onto the small plateau where a Grunt is in a Shade. See how much damage you can do from up there, with the chain-gun or whatever. You may like to pick up the rocket launcher first!
Actually, if you just want to get up there and you're not fussed about having a chain-gun with you, another way to get up is to fly there in a Banshee you left waiting for yourself by the door, as described in New toys in the first ground battle.