The best place I found to this where you can grind out the full 1000 in less than 20 minutes is on The Covenant. You're best just to leave this achievement until you've got all Halo 2 and Halo 3 achievements (and Halo 3:ODST if you have it).
Load up The Covenant on Halo 3 using Rally Point Bravo and playing on easy. Jump in the Hornet and head across to the landing zone. Around here you'll get a checkpoint allowing you to drop down into the two entrance areas and kill the 10-15 Brutes that are hanging around. Kill them all then restart the checkpoint.
I saved the clip grinding out the kills to help explain - skip to 2:50 to show the grind starting.