first off you'll need to know what fat scout is: fat scout (as i said before) is a sub-class for heavy in which you discard your minigun and mostly use your shotgun and melee
the best shotgun for the heavy is the family buniness. is has 8 round per mag and can deal some pretty nasty damage to your enemies. choice any melee you want too as it does not matter. same goes with the minigun as you will not use it.
here's a more detailed guide: https://steamcommunity.com/sharedfiles/filedetails/?id=2637652509