Khadgar, as an inexperienced if gifted apprentice mage, faces a bunch of orcs on his own in the swamp and manages to stand up to an orc warlock, who was likely far more experienced than him, until Medivh arrives and sets them all on fire.
Aegwynn's entire rampage in Northrend, shown in a vision of the past, where she tears apart the demons hunting the dragons, before she faces and defeats an avatar of Sargeras. Even if the last part was a part of his plan, it was still awesome.
As Lothar, Khadgar, and Garona make their way down into the dungeons beneath Karazhan they're confronted by huge demon hounds, one of which makes the first move by trying to pounce on Khadgar. Lothar immediately steps in the way, draws his massiveruned sword and cuts the beast in half up the middle in a single upwards swing.
The final confrontation with the possessed Medivh deep beneath Karazhan, where after having been forcibly aged into an old man and having witnessed Garona being Mind Raped and Lothar set on fire, Khadgar manages to stab Medivh in the chest with a sword when he's distracted. However, as Medivh dies, Sargeras takes full control of his body and is about to burn Khadgar to death; cue Lothar rising up and cutting offhis head.
When Thrall walks up to the man who taught him how to fight (after besieging his castle), grabs his earring (in response to the challenge he gives his students when they first meet to rip it off), then simply lets go of it and walks away.
Turalyon gets a big one in the novel adaptation of Tides of Darkness. After Orgrim Doomhammer kills Lothar, Turalyon's religious doubts finally click into place when he realizes that the orcs are not of his world. This lets him finally make full use of his Light-given powers and blinds all the nearby orcs. After that, he picks up Lothar's broken sword, beats Orgrim Doomhammer into the dust, before taking leadership of the Alliance army and finishing the battle.
Lothar and Doomhammer's one-on-one duel during the Battle of Blackrock Spire is a drawn-out brutal melee which pushes both combatants to their limits. It ends with Doomhammer breaking Lothar's sword and killing him, but not before Lothar uses said broken sword to deliver one last blow that almost kills the Warchief.
This particular fight was written by a guy who also works for Black Library, so of course it was awesome.
And the only reason Doomhammer won was because Lothar was already far out of it while Doomhammer was still in his prime, and it was still a close thing.
In the War Of The Ancients trilogy, Jarod Shadowsong, reluctant hero and leader of the mortal forces, willingly stands against Archimonde when challenged by him, with full knowledge that Archimonde could crush him like a bug.