If you're referring to their later duel on Draenor, I don't really think Garrosh won that, per se - sure he was able to knock Thrall down and stun him but he did so at the cost of his weapon and Thrall wasn't knocked out or otherwise incapacitated either. If Thrall hadn't opted to use his powers to pop Garrosh into the air then crush him with a hand of stone (followed by a lightning lobotomy) it's likely the two could've kept scrapping for some time. Not to mention that Garrosh never actually tags Thrall with Gorehowl at all in that duel, where he gets popped by the Doomhammer several times. Blow for blow I'd say Thrall was winning, and he's up on his feet after being body-slammed by Garrosh easily enough. It's actually not that great a showing for Garrosh when you actually watch the choreography of the fight, though I would say he's got a slight advantage before Thrall claps back with his Shamanism.
Shaman are also imbued with Elemental powers as part of their becoming Shaman, as well; if only by dint of their being part of the natural world themselves and able to tap into that connection to fuel their own basic powers. Not unlike Mages and their use of the Arcane, being able to use their own internal essence to cast spells (such as what Medivh does to Arcanagos). I also don't think the Mak'gora has had and fast rules the way most claim - it's not a codified ritual, but rather a term that makes conflict or combat sacrosanct. Orcs view any kind of personal duel through the cultural lens of the Mak'gora, such as how Saurfang views the fight between Sylvanas and Malfurion despite the fact that neither of the two are Orcs or share in his beliefs i.e. the Mak'gora. Malfurion is openly contemptuous of the idea, such as when Saurfang declares it as a stalling tactic in "A Good War." The only true rules for Mak'gora are those the combatants declare before fighting, such as the Mak'gora between Cairne and Garrosh, which was more elaborate and strict as it was a duel for the very leadership of the Horde.