If I saw an assault mech at this point they would be cleaning the inside of my lances cockpits and not of blood. 3x 50 or 55 tonners and 1 45t would not enjoy aa Assault mech. But am happy to know you dont have to take the story missions as they appear as was getting tough.
So use 2 as bait and 2 to get behind him.
Usually a Kintaro or Hunchback will quickly teach an assault not to turn their backs on smaller guys.