I thought I would put out there what worked for me for both of the 50,000 score meta game achievements (The Ark and The Covenant). Bear in mind, this method is quite slow and grindy. However, time and deaths aren't really a factor and you should clear 60,000 -70,000 points with no problem.
SKULLS: Tough Luck, Fog, Thunderstorm, and Mythic. These skulls give you a 6x scoring multiplier and are plenty for what you need. I added Cowbell, Grunt Birthday Party, and IWHBYD for fun.
DIFFICULTY: Normal. The above skulls will give the game a feel closer to Heroic difficulty. You'll die here and there but the penalty will not really hurt your score.
As I said, don't worry about rushing through the missions. They are very long and you probably wouldn't have gotten much of a time bonus anyway. Your goal is to kill/destroy everything. Headshots, multikills, and most of all enemy vehicles and turrets. Use a vehicle with firepower whenever you can. There is nothing wrong with grabbing a ghost and spamming it's gun at a wraith across the map. When in your Hornet, spam those rockets and clean up every Banshee in the air and AA gun on the ground from a distance. Also, when driving a Warthog remember that you get credit for your gunner's kills! Most important of all is to note that in either of these big missions you will get a Scorpion Tank to use. In the Ark mission this is clear but for the Covenant mission you could miss it if you rush. Right after clearing the third tower and getting ready to head to the double Scarab battle, there is a Warthog sitting out for you to take. Don't do it! Wait a few seconds and the UNSC will drop in a Scorpion too! Any opportunity to use a Tank, you take. Period.
Don't worry if you don't seem to have enough points and you're deep into the mission. Both The Ark and The Covenant and heavily backloaded points wise. I was down 10,000 to 15,000 points going into Rally Point Bravo for each mission and easily hit 70,000 points for each mission in the end. Good luck!