'No shots fired' apparently includes your knife, too, so basically leave your trigger alone completely. Don't even swing your knife in the air!
If you can get in a CTF game with just one other person you will stand a much better chance of getting this, because there are less people around to spot you when you sneak off with the flag. So, small team games are your best bet, as very few people still play them(at least not at off-peak hours).
Just make sure you choose a different route to the flag each time, or you will be picked off quickly. And this is a no-brainer, but if you do find yourself under fire, use the jump (A) and dodge (A+ left or right on the left stick) to full effect.
Boosting: jump in a small team game with another friend and try to get a plain old CTF match. Take each other's flag twice. Then one of you must take it once more for the match to end.