Well... it was hard to choose between Elites and Skirmishers. Mostly because you threw Firefight into the mix. Because of my past experiences with Elites on Firefight, it really was a very hard decision to make, what, with their constant dodging my sniper shots before I even fire. Makes them annoying to hit unless you have an armor-lock using teammate. Then they just stand still for some reason until I open fire. Anyway, I went with the Skirmishers. The things are just too fast! I can hardly hit them, even with melee! Seriously, it's like I'm about to kill the thing, and it suddenly sidesteps as I'm about to hit it/fire at it. That just absolutely takes the cake, right there, man. Very easily 20X worse than the Drones were, if you know how to deal with them.