Alright. The MK I mercs are the weakest, but they tend to travel in the largest groups, about nine or ten at a time. Simple explosives can deal with them with ease, MK IIs are somewhat harder to kill as they're armored, but there's about five to six a group, it's more than recommended to target the RPG user at his rocket with anything ranged. And MK IIIs are the most difficult, with many calibers being completely unable to get through it, there's only two per group, one will have a minigun. And the other will have a flamethrower. If you can't get any clear shots on the gas tank of the Pyro, deal with the faceplates, they're brittle against 6-gauge Buckshot.