Ooh, Granbull. One of my favorite Fairy-type Pokémon to use. With the introduction of the Fairy-type, I feel that Granbull, as well as the typing itself, has benefited overall. Granbull has a massive array of physical attacks at it's disposal, which is nice considering there aren't a whole lot of physical Fairy-type Pokémon, and most Fairy-type Pokémon generally don't have great coverage. Unfortunately, Attack is really the only good stat that Granbull has. Sure, it sports a base 90 HP stat, but this is offset by it's base 75 Defense and base 60 Special Defense. Even worse than that, however, is it's Speed, which sits at a base 45. Needless to say, Granbull won't be outspeeding really anything, unless you can get it set up through a Baton Passer, considering it doesn't learn any attacks that boost it's Speed (barring Z-Scary Face), and Quick Feet really doesn't do it too many favors in terms of boosting it's Speed or letting it survive. So, while Granbull may be a nice choice if you want a powerful physical attacker with an incredibly broad movepool, you're probably not going to see too much success from it in higher tiers, thanks to it's other low base stats.
Abilities
Intimidate: You know it, and we all love it. This is probably Granbull's best option for an ability, as it will soften physical hits from opponents, helping with it's low-ish bulk.
Quick Feet: This might sound good to help patch up it's terrible Speed stat, but remember two things. First, Quick Feet only gives a 50% boost, meaning that base 45 becomes ~67, which is still not great. Second, Granbull needs to be inflicted with a status condition, which is absolutely awful for a Pokémon with low survivability. Poison will gradually chip away more and more damage as time passes, Burn will cut it's Attack power, Paralysis will leave it with a chance of not being able to move, and Sleep and Freeze will render it completely immobile. Avoid running Quick Feet on your Granbull, if at all possible.
Rattled (Hidden Ability): This is better than Quick Feet, as it allows Granbull to gain a Speed buff every time it is hit with a Bug, Ghost, or Dark-type attack. On one hand, it resists both Bug and Dark-type moves, and since it is no longer a Normal-type, it can be hit by Ghost-type attacks. However, with Granbull's low-ish bulk, you're likely to see it faint before it can gain any momentum with this, and even getting one Speed boost on a switch-in won't do it too many favors. You're better off running Intimidate, end of discussion.
Items
Granbull doesn't have too many stat-boosting moves, so Choice Band might be a good option. You can also equip the Life Orb if you'd rather have the chance to get a Bulk Up or two in (good luck with that, though). Z-Crystals are also a viable option, namely Fairium Z for Play Rough. If you want to try something really weird, the Assault Vest will bring it's base 60 Special Defense to 90, which complements it's HP a little better. Still, your best bets are either Choice Band or Life Orb.
Moves
As I mentioned earlier, Granbull has a massive movepool that is complemented by it's base 120 Attack stat. Play Rough is Granbull's only reliable source for STAB, so you'll likely want to run this no matter what. I've found that Wild Charge (TM), Earthquake (TM), and Crunch are good options for it, as they all boast decent power. Other high-powered options include Outrage, Stone Edge (TM), Close Combat (egg), Double Edge (egg), and Superpower (tutor). If you find yourself worried about the recoil damage from Wild Charge and Double Edge, Granbull also learns the elemental fang attacks, as well as Return (TM). Bulk Up (TM) is one of Granbull's only boosting moves, and it's most reliable, from what I have been able to see. If you plan on using a Z-Crystal with Granbull, Heal Bell (egg/tutor) is an option if you want to get a full recovery with Z-Heal Bell.
Effort Values and Nature
In order to maximize it's destructive power, an Adamant nature is preferable, although a Brave nature can be used to help optimize Trick Room support. With these abilities, I would recommend full EV investment in Attack, with the rest going into HP and Defense in quantities based on what you expect Granbull to be facing against. An Impish nature could help shore up it's low Defense, but even with a perfect IV, it only reaches 273 with full investment, so I'd advise against it, in favor of Adamant or Brave.
Partners
With a base 45 Speed stat, Granbull loves Trick Room support. Bulky Trick Room setters such as Slowbro, Eviolite Porygon2, Cresselia, and Stakataka seem like decent options out of what I found. Granbull might appreciate faster teammates and priority users that can pick off and dispose of other fast opponents, in order to allow it time to do it's job. Coverage doesn't seem like too big of an issue to me, seeing as both of Granbull's weakness + the other type (Fire) that resists it's STAB can be dealt with via Earthquake (or another move, if a dual-type combo presents a neutral effectiveness or immunity against Earthquake).
TL;DR
Granbull's impressive Attack and huge movepool make it a great choice for a sweeper of sorts in lower tiers. Unfortunately, it's other stats don't quite do it any favors, so moderate to heavy team support is necessary to allow it to function as well as it possibly can.