I just played through the first 15 after first getting to wave 6, only to misread "Restart checkpoint" as "Restart wave". Don't do that, ever! I got the impression this is gonna be a tedious DLC.
For 15, I didn't get a challenge failed when I damaged either enemy with my weapons. I guess you only need to finish them off with a vigor? Unfortunately I died so I couldn't get the ribbon. The handyman standing in the middle of the map just seemed to stand there, and I could follow the skyrail to the elevated area opposite of the RPG and peek around the corner and snipe him in the heart. After killing him the motorized patriot was a piece of cake, and the finally the final handyman is easy to kill as long as you jump on/off the skyrail. He'll continuously electrocute the skyrail which allows easy damage towards him.
If him standing still was a bug, and the handyman in the middle is really moving around, then I have no idea yet how to do that ribbon.
Dec 23rd 2011 | 111,111
Feb 6th 2012 | 123,456
Feb 25th 2012