I did it all in Core TDM, so I feel inclined to try to help.
The double kill. This can be done actually fairly easy with the Tri-Bolt attachment, but it still takes your enemies being bunched together. Better yet, try it in Hardcore where the bolt is an instant kill and the explosion is likely to take out another person anyway.
Kill with each "round" in your "clip". This basically takes precise aiming. The worst thing here is to stick someone with a crossbow bolt and have a teammate kill them before they die. All in all, you'll probably get it while trying to get -
The Bloodthirsty. The f***ing Bloodthirsty. This isn't easy. Apparently, you don't need to get the 5 kills in a row - you just need to get 5 kills with the Crossbow in one life. This means you can put on a primary weapon and Scavenger. Don't worry too much about missing, just try to get a couple of kills before you run out of ammo. Pick up some scavenger packs, using your primary weapon if you need to, reload and continue. If the word is true, then you'll get the challenge anyway.
If not, then I dunno. I just camping it out and perfectly executed every kill without missing, and still had one bolt to spare. Using Tri-Bolt and Scavenger might help if you want to go into Hardcore and kill people with direct impact.
In any case, getting the crossbow gold took me over a week of effort.