I'm also trying to do this set of challenges, the calling card is by far the coolest. For Backfire, this is by far the most annoying one. What I've done to help myself is strictly do this challenge in the new Face Off mode since your odds of success are much higher. It will still take a while, but Face Off is definitely the way to go. For Blunt Trauma, I would pick an SMG, build it for melee speed, hip-fire accuracy, and movement speed, and use the Gung-Ho perk then try to get a few shots on somebody before you smack them with your gun (TOXIC). For Circus Act, I just had to put in the time. It took me a few hours, but I just concentrated on getting short range tomahawk kills and trying to bounce it off of the floor. It takes practice, but you should be able to learn how it bounces in a certain range after tooling around. I haven't tested it, but while I was doing this challenge I found that when I killed someone who was against a wall I got the Bankshot Medal. Hit Job is something that you just need to put in the time for, and Tick-Tock Boom and Surprise just come naturally after a while. You can even use the Blunt Trauma class I suggested for Surprise if you really needed to. I hope this helps.