I honestly didn't think it was possible, isn't the challenge to get assists?

Just spam the plasma pistol.

Don't charge it up. Only charge a shot if you are going to weaken shield and switch to another weapon to finish off, or if you are trying to disable a vehicle.

The quickest way I've seen and done it is to just tap your right trigger so it spams small shots at them. It is possible. It was hard as hell in the other Halos, but Halo 4 plasma pistol kills are relatively easy, if you can aim.

When spamming, spam at a medium to short range. By medium I mean about the distance of 2 warthogs to as close as you can get without them meleeing you.

Haven't played in a while so I don't know what playlist to tell you, but I would probably go with the playlist that has mainly noobs or an objective game.

Like oddball, ctf, or king of the hill. While the enemy is approaching the objective lob a nade in to weaken and spam your plasma pistol. Be sneaky and make sure you get a few shots in though. Still takes a couple shots to kill even with no shields on.