This one is actually pretty easy. Just make a gametype where you have infinite ammo without having to reload and you spawn with a rocket launcher. It also helps if you do 300% damage and they can only take 10%. Set all enemies to be grunts and play on the map Corvette.
Let all the grunts drop down from the ships bringing them in and start shooting. It might take a couple of tries since the grunts tend to split up into smaller groups.
For a good gametype, download this one: http://www.bungie.net/stats/reach/filedetails.aspx?fid=2007&...
All credits for the gametype goes to Rabidkitty4