Just a simple poll, which enemy faction do you absolutely hate going up against?
For me, it's the cleaners. Nicking their tanks, seeing the jet of flame and knowing their about to 'take off' (as hilarious as that can be when it happens) doesn't quite make up for the flamethrowers, molotovs and guys with shields-and-fireaxes.
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I'm not terribly far (level 14 ish) but I loathe cleaner bosses. My usual tactic is to knock out the mobs and just out maneuver them by running/flanking