Halo 3

The easiest way to pull this achievement off is to go through the whole stage in Normal. Turning on skulls will be needed, and the best route to go is Catch, Tough Luck, Fog, and Famine. If you use this and make sure to kill as many vehicles as possible, you can expect to pull it off as long as you die less than 30-40 times (How many kills and medals you get affects this. I died 14 times, had 44 medals, and got 61,508 points). If you can do the whole stage in less than 50 minutes, there is a 1.5x bonus for you, but it's not necessary. Here's a few tips though:

Never rush. With Catch on, enemies will much more frequently throw grenades. You'll really only see this effect on grunts and brutes, but you can expect most of them to toss at least one grenade at you on sight.

Get in the turret of the Warthog during the first vehicle sequence. Your driver may be horrible, but you will pick up some points this way. Make sure to shoot the Wraith and Shades (the Turret Balls) before you go after specific enemies.

When you see a drop pod with weapons in it, restock from there. Weapons in the equipment pods are NOT affected by Famine, and therefore have full ammo.

In the last part of the stage, let your team do the killing before you instead of charging straight ahead. They will sweep up a number of enemies, and lessen the odds of you getting killed yourself. Once you're inside the heart of the Ark, there are weapons anyways so don't really get too concerned about conserving ammo. There will also be Spikers in equipment pods at the doorways.

You may hit 50,000 partway through the stage. If this happens, take things easy. The only way for you to lose the achievement now is to drop below 50,000 due to too many deaths.

Did it alone with iron, fog and luck (6x bonus) and Grunt Party (no bonus but you can save ammo) on normal difficulty. Be patient no need of time bonus to accomplish mission. The Ghost is better than the warthog (you drive and shoot) and be very careful when destroying the couple of scarabs (when you destroy the first a prowler and two ghosts will appear).

Good Luck!

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!

Hey guys here's a solution that helped me...I used Tough Luck, Fog, Famine, Thunderstorm and Mythic and I had an 8.5 multiplier (I also used Grunt Birthday Party and IWHBYD but they don't count towards your multiplier)

I ended up getting around 74,000 as my final score and died a lot...so if you're struggling it may help to try this method...I warn you this isn't an easy achievement but with this method you have a lot of leeway as I died a lot during the Scarab part and still managed to get well over 50,000 points

For this achievement you'll need to earn at least 50,000 scoring points on the seventh mission of the Campaign, The Covenant. This can be done solo or with a few buddies. (I got all mine solo and using the method below)

First you'll need to enable either Free-For-All or cooperative scoring from the campaign options. Then select the following for quickest results:

Normal Difficulty
Catch Skull - (1.00 multiplier)
Fog Skull - (1.00 Multiplier)
Famine Skull - (1.00 Multiplier)

So you'll be playing on Normal difficulty with enemies chucking nades alot, no Mini-Map, and ammo pick-ups are scarce. However, being on Normal difficulty will make this a cakewalk. And having these 3 skulls on will start your kill scoring at a x3 Multiplier, so as long as you finish the mission within a reasonable time, and die little-to-no times you'll have more than enough points to pop your achievement!