I've always had gnolls be big demon worshippers - and slowly transitioned them into being fiends themselves. Technically, I still have humanoid gnolls in my game to address some continuity issues, but for the most part all gnolls are now low level fiends.
While you may encounter them as individuals, this is rare. They almost always engage in combat as swarms/mobs.
When they go into battle they enter a hysterical rage. This rage automatically causes creatures that hear that cacophany to bleed from the eyes and grin uncontrollably. They also can't shut their mouth or close their eyes while they hear the raging laughter. In addition, if you enter within 5 feet of a raging mob of gnolls you have to make a wisdom saving throw (DC 9 only) or you'll succumb to Uncontrollable Hideous Laughter (as the spell). On the plus side, they are so reckless that they automatically provoke opportunity attacks when they enter a square within your reach. They also heal fairly rapidly - automatically healing 10% of their maximum hps every hour.
As Demonic Fiends, the Gnolls are more of a force of nature than thinking beings. They follow certain priorities and are nearly as mindless as the undead in executing them. They have almost no sense of self preservation. First - feast. They kill whenever possible and eat what they kill immediately. That means that if you drop in combat, they will consume your downed PC. Second - scare. They find joy in seeing fear in intelligent beings. As such, they make a show of how feral they are, and relish in hunting down those that are fleeing from them. This makes their priority in combat: 1.) Target those closest to death first (whether wounded or weak), and then those that flee or move away from the gnolls over those that want to meet them face to face.
Gnoll archers use special 'harpoon' bows that shoot oversized arrows. The oversized arrows are poisoned and drag behind them filaments of specially prepared webs that will drag behind a target and effectively grapple them to a spot - locking them down to a spot unless they can beat a low DC (10 generally) on athletics or acrobatics (doesn't require an action, but attempted the first time you try to move and if you fail your speed drops to zero). Because they work as a mob, they make a bunch of ranged attacks. However, they rarely fire twice in a combat as they prefer to close in and consume what they slow down. In fact, most gnoll mobs only carry one shot.
Gnoll leaders are more intelligent, but equally bloodthirsty. They primarily direct the swarms to give them extra attacks, or to allow them to do more complex maneuvering.
My only spellcasting gnolls are psionic in nature and use powers that cause auras and zones of fear, malice, hate and insanity. They also project auras that allow allies to absorb blood in order to heal. Typically, they are just one of the many gnolls in a mob and their presence enhances the mob.
There are Clerics of Yeenoghu, but they are typically survivors of gnoll raids who are captured by a clergy and indoctrinated viciously. As such, they can be of any heritage/race. One adventure had gnolls overtake a fey encampment and I used a sprite/pixie cleric as a war leader - that was amusing. [This was inspired by the Reavers of Firefly Lore]
The best way to combat gnolls is to execute their leaders and then capitalize upon their predictability to guide them in ways that minimize their damage. My more experienced players treat them like hazzards rather than monsters and can often turn them into useful tools of destruction rather than foes by luring them into conflict with other foes that seem weaker than the PCs. Illusions are particularly useful in these attempts.