The Ark.. as usual, Normal is best, choose 3 skulls of your choice (must be gold). run through, kill everything in your path, if possible get some marines into a vehicle with you, make sure you're the driver, you'll get the marine kill points as well as your splatters. careful with the rockets and sniper rounds, you may need them as you go on. aparentally there's a dead marine next to a warthog with a sniper in the part before you take out the 2 AA wraiths.. so far, i haven't found that ammo.. there's a chopper nearby that you can use for longrange attacks, get in it, take it to the top of the hill, and let loose with the cannons, best bet is to take out the wraith, the ghosts, the turret, the towers, and then any stragglers left behind. remember, a grunt with grenades is deadly. the rest of the level up to the scarab should be easy, best bet if you stick with the warthog is to start doing laps, go down close to the first wraith, but not in range of the turret, then drive back to the tanks, repeat. if you're a tanker, hijack a tank and let loose on your enemies like there's no tomorrow. the scarab should be easy enough to take out. and the rest of the level is pretty easy as well, use grenades, BR, mauler and any other weapons you may have.
hope this helps, if you don't find this helpful, then don't bother looking at the forums for suggestions.
I think Heroic is the best choice, and have 3-4 gold skulls on. Because if you can get to the citadell (after the two scarabs have been killed) you will have a bit more than 70K and the enemies inside the citadell is really easy, the only "hard" part is the hammer brute (and maby the fule rod grunts at the door) but a well placed rocket and your good. BTW if you got 70K+ you can die about 80+ times (I think you get 250- if you die in Heroic, don't really remember, thou if you get a lower reduction (say 150) you can die 130+ times) and this makes its impossible to not get it. I hope that this helps you if you have a hard time getting the achievement.
BTW when I got it i had "failed" to get it like 7, or more, times before. This was because when I came to the cinematicts in the citadell (right after the profet says "stand ferm" the game restarted from the mission from the start -.-' guess if I got MAD!!!