Hello to anyone reading this. I have an easier way for assists that can even get you some sprees along the way. Load up The Package Rally Point Bravo on Legendary (with Mythic and Thunderstorm) with 2 controllers. Have the guest (or if you get someone to help you) wait where you would normally get your checkpoint. The first person (who wants the assists) goes to the grunts on the bridge and shoots the first 2 one time to slow them down, then very quickly run back to the checkpoint spot. You SHOULD get a checkpoint, if not, then set it up again. Have the person who needs the assists throw a grenade at then (if you want the assist spree, then throw one at the front of the group and one at the back of the group, this should get you 5-7 assists) then rinse and repeat. I got my assists to silver by myself in about 40 minutes (including different trials and setups). It'd go a lot faster if you had someone getting the grenade kills with you, too. Anyways, to people who are looking at the post to ask if you get banned for doing this, no.
NO YOU DON'T GET BANNED. STOP ASKING. PERIOD.
[Edited on 07.21.2011 3:57 AM PDT]