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 Chasm Ten map on Heroic and wander around the upper catwalks close to the vents and doors. These are useful for flanking enemies and getting into cover. On the first rounds, use the sniper located in the upper tunnel-room to kill grunts and get a sharpshooter if possible (spree medals give lots of points). In general, 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 in the spawn point on the bottom level of the map. On the bonus rounds at the end of each set, use your pistol to get easy headshots on grunts, but don't ever stop moving. Bonus round grunts are grenade-happy and dangerous.
Other than that, simply survive until you hit the 200,000 points required for this achievement. I've personally survived until just over 400,000 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.