After getting the bikes and uniforms, you need to steal a truck and repaint it. You have to go fly an Air Crane and steal the truck. To get the Air Crane, you need to steal a military vehicle and go to the fuel dump in Northeastern Las Venturas. If you go to the fuel dump, a Patriot (hum-vee) will drive out. You can try to force your way past it (which will trigger combat with the military ) or try to jack the Patriot and make your own way in. When you approach the air crane, two hunter choppers will approach, and you must get on an emplaced gun to take them down.
Then, take the Air Crane, fly to the depot in the southeast, and pick up the truck. Fly it out to the Verdant Meadows airfield, drop the truck in the indicated area, and then land.
There is another way of dealing with this mission if you don't want to go in by the front gate. Go to the airport and grab a plane. Fly up to a high elevation, and fly to the compound, when you get close, jump out and use your parachute to land on the roof where the helicopter is (rockets will shoot at you, but they will miss because they can't home in on a parachute). From there, go to the stationary gun and take down the hunters that come. Afterward, take the helicopter and continue with the mission like you were supposed to do.
Another way to get to the helicopter is with the jetpack. Make sure you have a weapon that can be wielded from the jetpack, such as the Micro SMG. You can approach the helipad from the east and catch the guards by surprise. Or, you can use the sniper rifle from a distance.
Another option is to get yourself a helicopter. Any 'copter will do, but the Seasparrow is nice, because it has the front firing machine gun. Approach the helipad on the base low, from the south, and the men firing rockets at you from the helipad will miss, because they are on the north edge of the helipad. You can land on the helipad itself, or just to the side of it. If you make sure you bring the guided rocket launcher with you for this mission, it makes taking out the Hunters very easy, without using the gun emplacements.
Once you finish the mission you will have this Leviathan (without the magnet) spawn at your airstrip. If you have gotten all gold in the Learning to Fly training missions, this will pose a problem, for occasionally when you come to your airstrip, there will be a Hunter helicopter, and sometimes there will be a Leviathan. Both the Hunter and Leviathan want to spawn in the same place.