Check out MythicTyrant’s legendary guide for beating Halo 3 with no deaths. Do note that the video quality of the first mission isn’t the best, but from there on out, the quality is good.
Link: Sierra 117
Link: Crow’s Nest
Link: Tsavo Highway
Link: The Storm
Link: Floodgate
Link: The Ark
Link: The Covenant
Link: Cortana
Link: Halo