Dark Sector

Easiest I've found for boosting this achievement is with 2 people.. have both stand near the fire and throw your glaive into the flames. At the same time (or as near as possible) push in your right thumbstick to explode the glaive. You can get close to 300 points each if you keep this method up for an hour match.

EDIT: As mentioned by Lozzy Denton, try to be sure you are both looking at each other.. makes it easier so you can time the explosions when you can see if you both have the glaive ready to go.

There is also an alternative method to this, which I believe to be a bit faster and a little less boring.

It requires 3 people instead of 2. Here's the breakdown:

Someone host a RANKED epidemic match on the Court map (4 people, 20 score limit, 30 minutes)

Second person join (this will be the person getting points)

Third person joins (1st and 3rd person will be on one team, 2nd by himself)

The two people on the same team will just stand at the spawn, the single person will run to the other spawn (make sure and throw your glaive in the fire on the way)

Now simply use a finisher on the INFECTED guy, then click the right stick to explode your glaive, killing the normal guy.

For some reason doing this will net you 6 points, repeat for 20 rounds and you will get between 110-120 points each time, which takes roughly 8-10 minutes. Then simply switch off who goes on the team by themselves.