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.