To use Granbull, you need to abuse all of his finer points while working around his downsides.
What does he have? Intimidate, 120 base attack, Heal Bell, Normal Typing. Low speed but in a Trick Room Team, it's an advantage.
Brave - Intimidate
252 Atk - 252 HP - 4 Def
Intimidate users have an easy time switching in and doing something, like Salamence back in Gen 4. Return is your main STAB, which deals insane damage from a Choice Band. Retaliate hurts even more when Revenge Killing, which you will commonly do in a Trick Room Team. Since you are on a timer, utilizing Destiny Bond and Explosion to deal quick KOs can make Retaliate effective, provided you have removed Ghosts. Even then, Ghosts will hate a properly predicted Crunch. Even max Def/HP Cofagrigus is 2HKO'd after Rocks. Close Combat is your strongest Fighting Move. The Defense drop may seem unappealing, but Intimidate remedies that for the turn. Fire Punch is an option if you don't have suicidal teammates.
Relaxed - Intimidate
252 HP - 252 Def - 4 Atk
A more defensive Granbull. This Granbull doesn't have to fear status as he can just ring them away with Heal Bell. The missing Attack stat is remedied with Super Fang, which softens up opponents for yourself or for your teammates. Return and Crunch are your offensive options. While a move to hit Steels harder would be preferable, Super Fang stills hits them which softens them up for a teammate. Crunch is just for Ghosts.
This set is also effective in Doubles, where Intimidate and Heal Bell can save suffering Trick Room users. Try it with a RestRoom (LOL) user.