Load Tip of the Spear, starting from rally point Bravo on at least Normal difficulty. Here's a clip of mine:
And here's a text guide:
Casually start following the objectives. Eventually you'll come up to a big building with the BXR logo, which will also unlock the following achievement if you look at it: Leave the Warthog and
go up the catwalk. After a small turret which is usually operated by an Elite, you'll turn left and in front of you you'll find a weapon rack with Assault Rifles. Moving to the left of it and going down a very small flight of stairs, you'll see a slightly shorter building in front of you with a hole in its roof. On that roof are some enemies, but most importantly there's an Elite you need to kill, he'll also have a red objective arrow on him. You can sprint and crouch jump to get on the roof or shoot him from where you are. Once he's dead, the achievement will pop. If you don't make the jump or the Elite doesn't die by your hands, just restart from checkpoint.