I just completed "A monument to all your sins" last night, and I have to admit, it was tough. However, here are a few simple tips to guarantee your achievement. 1) Be Patient. Move slowly through these missions. Clear out enemies from a distance. You'll be much safer. 2) Don't be afraid of the plasma pistol. This thing saved my skin more times than I can count. They're practically everywhere, too, so you won't be without one when you run into those Ultra Elites. The same goes for the needler. They take out Brutes very easily. 3) Conserve your ammo. Don't waste an entire DMR clip on two grunts. Aim carefully, and use a pistol if you have one. There are plenty of Assault Rifles and shotguns, but DMRs are hard to find in the campaign. 4) Armor Abilities. It really depends on what kind of player you are, but I got the most use out of the Drop shield and Sprint. Sprint will actually get you through small groups of enemies and let you tell the tale. However, the Drop Shield is the best thing I've ever used. When deployed it heals you, and recharges your shields. No more looking around for a health pack. You've got one equipped already! 5) Finally, take your advanced enemies out one by one. Those last three elites on "The Pillar of Autumn" are awful if you try to take more than one out at a time. Aim carefully, use your plasma pistol, and be careful. I certainly hope these tips help! Good luck, and Godspeed.