Don't be scared off this huge score. This is actually quite easy to achieve by starting up the 13th mission (Genesis) on heroic difficulty for Co-Op. It is easier to have just you and a friend instead of a fire team of 4, but both are doable. Then select the skulls tough luck, catch, fog, famine, and tilt. You will all of these skulls on be because there is a method to stay in the tank for the entire mission.
Once you have 200,000 points, let your friend get 200,000 points and then go from there. :)
I am adding a link below to show you how you can get the tank to the last warden fight where you would have to usually fight him on ground. Just stay back throughout the mission and take your time. If you die, restart checkpoint so you still have the tank. All credit for the glitch goes to Aeons of Blight
(Glitch still works I tried it this morning, it may take a few times, but it definitely works). https://m.youtube.com/watch?v=E8zvuXEKz6I
Also note; when you get to the first warden, you must exit the tank and walk across the bridge to trigger the warden. Then run back to the tank and have at it.