Re: Trip wire bomb kills
There isn't really a super easy way to get the kills, it just takes time and patience.
Sneak up behind enemies (camping snipers are good for this) and plant one behind them. Shoot them with your pistol and when they turn around they trigger the bomb.
Plant them in choke points or areas that you know will be contested throughout the match. I've gotten a few good double kills this way. The hardest part isn't really about enemies running into the tripwires but making sure that they are softened up enough beforehand so that the bomb actually kills them.
Think about spots that you would use to take cover! Plant bombs there and when an enemy is getting shot up and dives for cover he'll run into your bomb.
Or, if you can at least get 1-2 lucky kill(s) a day then that'll only take about two weeks. Just plant them all over and enjoy the game! Don't worry about the assignment too much!