I personally hate Shotgunners because they'll just rush you whenever they feel like it and do so much damage.
Grenaders are annoying as well, cause they throw too damn much!
And don't even get me started on Melee guys. People who did the Lincoln Tunnel challenge mode know what I mean.
Edit: I recently tried the WarrenGate Power Plant challenge mission...it's fuking impossible. It needs fixing. Especially the timed parts, like how are you supposed to beat 3 waves of shotgunners, grenaders, and heavy gunners in 8 minutes when it takes a whopping 3 minutes to clear one wave!? Wtf Ubisoft?! >:(
Edit2: I have now been told that the boss room consists of a 3rd full shotgunners wave. THAT FUKING MISSION NEEDS REDOING. Dammit Ubisoft...
And let's not forget Alex. ;)
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As for the Warrengate challenging, gear up more, first time I did it we had +/- 10 sec left. Next time it cane as challenging daily we had over 3 minutes left of the timer. Challenging is a joke now.