A team of four works great, but a single person can plan out kills better than a group of four. For example, if four players kill one grunt each, they wont get as much as one player who gets an overkill on them all.
So, if you're decent at Halo, you can play the Alpha Site map on Heroic and sit on the right-hand side of the "safe" part of the map. If needed, there are health packs located up the stairs in the back of the map as well as ammo/grenades/health packs on the sides of the same stairs. On the first rounds, use the sniper located on one of the walls to kill grunts and get a sharpshooter if possible (spree medals give lots of points). Use plasma pistol/pistol combo to kill brutes and try to line up higher multikills for more points. On the fifth wave, use a plasma pistol to knock off the shields of either the fuel rod brutes, or the hammer brute's shields, and finish them off with a rocket. The rocket launcher is on the center wall of the "unsafe" part of the map. On the bonus rounds at the end of each set, it is possible to headshot a grunt in the vents they come out of as soon as it spawns, and get over 30,000 points from a single shot. This happens when you have the Grunt Birthday Party skull on and they literally blow each other up and get you plenty of points, depending on what kind of a multiplier you have.
Other than that, simply survive until you hit the 200,000 points required for this achievement. I've personally survived until just over 1,000,000 points playing under these very similar guidelines, so it is not overly difficult if you're a good shot.
*Note: If you can't survive well solo, play with a friend or two, but it might take a little longer.
**Note: Similar strategies to this can be used on the other Firefight maps as well.