You must have DLC 2 for this.

First destroy all the FBI cars and when you see a black Oppressor don't shoot it, because it is the Ultor Oppressor.

If you see men shooting at you leave them. When you're about to shoot down the Ultor Tornado the helicopter will go down. But it will still work.

After that go in the helicopter and when you see the Ultor Oppressor go near it, then abit high up and jump out of the helicopter. Parachute onto the Ultor Oppressor.

When you're on it pess Y and you will get in the Ultor Oppressor.

After that pick up Gryphon and go abit low. When you are at the hide out and if you see a blue circle land on it and you will complete the mission.

You will then have the Ultor Oppressor and you can store it at your garage.