Frankly the best set Granbull has going for it is as a Physical Tank/Supporter. Sure he's got attack. But he's faaaaar too slow to abuse it. Play on his Strengths: Good defensive typing. Buffered Defense thanks to Intimidate , good HP stat and passable defense (turns into base ~120 Def on the switch in off the top of my head thanks to lowering of defense on opponent pokemon). Passible support movepool consisting of thunder wave, reflect, roar and most importantly, the semi rare heal bell. So yeah, like this:
Granbull @ Leftovers
252 HP, 252 Def, 4 Spdef/Atk
Heal Bell is the defining move of the set. 252 HP and Defense, intimidate and leftovers to maximize tanking potential. Play Rough for STAB. Earthquake is a great move to pair with Play rough, only walled by flying/levitating fire, steel and poison types. And then select the other supportive move of choice. Thunder wave can slow down opponents to aid a team mate to sweep, roar to phaze to prevent set-ups and potentially rack up hazard damage, and finally reflect to boost the defense of team mates if you need it.
Beautifly are my Bishie!
hands off or my Beautifly's shall take you away and have their way with you!