Go right through the front door using Trevor and begin assault. Utilize his special ability until you can make it to cover. Circle right around the mill while following Franklin. Hide from cover as much as you can and watch out the catwalks.
During the snipers' shootout, swap to Michael and sweep off the roof, then return to Trevor.
Blast your way outside using the same procedure you went in. Smash the vehicles, swap to Michael and clean out the front of the mill.
Get the truck as Franklin and head home.