Ok, to boost this you need to have one peson whose at least Level 31 so they can equip it in their loadout.
For rank boosting, the two best gammodes are either Classic Conflict or Heroes.
For Heroes XP boosting, see my guide in the Boys with Toys achievement.
Even though it's specifically set up for boosting that achievement, the XP you earn just following the process is quite substantial (a boosting party of 5 can net one or two players close to 10k XP each match)
To rank boost in Classic Conflict, pick an easy map like Jungle, Nightclub or Solar and have everyone suicide on the one player who needs to rank up. Try to mix this up with kill streaks and headshots, especially if the rank boosting player has accolades available, try to pull as many of these off as you can. Even without accolades a match will take about 2 minutes and get anywhere from 6-10k XP a match.
Once you've got your guy at level 31, keep playing classic conflict and continue to suicide as you've done to rank up, EXCEPT, this time, the rank 31 will be killed by a random player who CAN AND WILL pick up his KL033 for 20 kills in a match. Rinse and Repeat 3 more times to get your 64 kills, then have the kl033 wielder killed and pass on the mantle to another. Round robin the kl033 until all players have the achievement.
If you stick to small maps and everyone suicides on the centre then you'll look at about 2-3 minutes a game. It takes 3.2 games pop the achievement assuming all 20 are KL033 melee kills (which is why for sanity's sake I recommend that whomever kills the level 31 for the kl033 is NOT the one boosting the achievement. Just to keep the numbers straight). Each person should pop the achievement within 10 minutes of boosting using this method.