Better aim would be a suggestion. i beat half the campaign solo legendary on the first day but gave up since mission 5 was too hard for me at the time. After playing matchmaking for a month or two and improving my aim, the mission wasn't that hard. Basically, make sure you have good accuracy especially with the sniper on that level and don't expect to rambo everything.
Other tips would be to use the rocket turrets on the mammoth and take out all the phantoms first then use your br/precision weapon and rockets to take out all the ghosts/wraiths/banshees.