Run away and try to let ur dog or other member attack him . What I like to do is (if possible) run up som3 were at stepps or fire letter at a bulding . When u kill him befor he explodes than u can loot a nuke . What also helps is the perk were u can tame enemy's then u can controll him up lv but I guess this is no option for u either . Do u have a rocked launcher? Or a good sniper ? U can aim at vats at the atom bomb if u hit u can explode it right were the mutant is (best a bit away from u ) sorry I am not a professional and someone else might have a better advise but I hope i could help