Warthog
The green off-road vehicle is not only a staple of the game's human faction, but is famous on its own as an indestructible jeep-like vehicle. Start by looking at photos of the vehicle, such as the ones listed under the References and Resources. To build one, you'll need some large, off-road-style wheels and a small, square wheelbase. Build up some green side walls, two front seats for the driver and passenger and install a turret on the back--armed with a shielded gun. The sides of the vehicle should be 1-brick high around the driver's seats and about 2 around the turret base. The best turret bases are made from 4-stud by 4-stud rotating bricks. Use a hinge piece to tilt the nose forward, and add the characteristic tusks and winch on the front. Covering the front and back with connected roll bars and adding a windshield complete the vehicle.
Banshee
The Covenant Banshee fighter is a fast-moving aerial fighter which, despite being an enemy vehicle, is often commandeered by Master Chief during his travels, or players during their online matches. The vehicle is typically purple in the game, but as LEGO doesn't have any purple bricks, the closest substitute is blue. The fuselage of the vehicle should be long and narrow, with a slight curve created by staggering plates on the bottom. Most builders start with a base around 2-studs wide and build up to no more than 4-studs, except where the wings are placed. The front of the vehicle needs to be on a hinge, to accommodate your characters getting in and out. The fighter's cockpit is on the rear of the craft, where the pilot lies prone on the vehicle and pilots it this way, looking through a viewing port on the front. For a number of the details, you'll need to find pieces with studs on the sides, and two sturdy hinge pieces needed to attach the wings at the downward angle. The Banshee's design also includes two smaller, sub-wings which point from the wing tips toward the rear of the ship. For more design tips, see the second Reference or the links under Resources.
UNSC and Covenant Forces
No battle would be complete without some Spartan warriors and alien Elites to battle. For the human side, the challenge is recreating the armor on the same scale as LEGO Minifigs. Some builders use decals, some of which are available for purchase under the second Reference. For a simplified look, Minifigs with green, red or blue torsos and matching helmets are fairly close. The Elites have to be built from as many small, hook-shaped pieces as you can find. These aliens have elongated feet, which place what would be ankles near knee height, and shortened calves and thighs, placing their knees nearer to hip level. The head is an elongated skull with four mandibles at the mouth, and a slim neck. Use your imagination, or examine a more detailed set of instructions under the third Reference.