Hell, it isn't THAT hard. I'll admit, legendary on halo 2 compared to halo is insane, but it isn't that hard. I have not beaten it myself, and the last boss is a BITCH on legendary(if you don't know the trick to killing him), but it isn't so hard that you can't beat 3 grunts(unless they are using needlers which is a bitch and sometimes can get you killed.)

Just be sneaky and don't try to be rambo. Legendary is all about taking them out one at a time and being slow/sneaky.