Reach's 4th Mission, Nightfall, is a bit different than many levels in Halo: Reach in that you can finish this rather stealthily. The only other level I can think of that is similar is Truth and Reconciliation from Halo: CE.
You'll work through this level with Jun and are provided a sniper rifle from the beginning.
I'm not going to give a guide to completing this level since on normal this isn't that hard. I will say that almost every section can be skipped. There is only one spot in the level that prevents you from moving. At first glance you'd think you have to stick around and fight your way so that this door opens, but there's another way around it.
There is a well known shortcut that can save some time and fighting. When you get to the section where there's a door blocking your path it is possible to drive a forklift at the center of the door. When you exit the forklift you will jump through the door and end up on the other side. This can take some practice to get the correct location. No Covenant will spawn anymore so it's possible to just sprint to the end. This is useful on Legendary and if you're doing speed runs. I've included a video below. I take no credit for its creation.
The To War achievement in Halo: Reach worth 12 pointsComplete the 5th mission on Normal or harder.