MK Is are the most common and serve as baseline for what the other marks are. They come with light armor and unprotected spots in armor. They travel in groups of nine, but they stay clustered together to prevent them from being picked apart one by one.
MK IIs are essentially shown in the Gif, with armor enhancements and more militarized equipment and better strategies. To kill one with overwhelmed and ineffective weaponry like pistols, you must force the MK IIs head to a side and fire at the exposed neck. They have a squad leader who carries an RPG-7, and they travel in smaller groups, specifically groups of seven men.
MK IIIs are the introduction of mercenaries carrying dangerous equipment, brandishing armor that makes it useless to fire pistols at them, only Shotguns and Bolt Actions can break the armor normally. MK IIIs carry either Miniguns or Flamethrowers, each squad has two men, one minigunner and one flametrooper.
MK IVs are the serious threat, with a heavily reinforced Minigun and immunity to 12 gauge shotguns, they can effectively ruin anything. Their downside is that they're slow and they can't jump to ledges as effectively as their baselines. They're limited by another factor: There can only be one at a time. And when they're on fire, the fire can't be put out as easily.
There's rumors that an MK V mercenary was existent, but no evidence yet.
