It would have to come from entitlements - Soc. Security, Medicare and Medicaid. They are by far the biggest portion of money spent by Fedzilla. Eliminating EPA, Depts of Commerce, Education, Energy would save very little - although in terms of eliminating regulation, it would probably stimulate economic growth quite a lot.
Slashing the federal government by 80% would theoretically leave the DOD untouched becasue the DOD is only 1/8th of the budget. Of course what should happen also is streamlining DOD the way Reagan did, getting out the waste and getting in the best and the brightest to defend our country.