To be honest, I've have always thought it was WOW TBC Gold because of garrosh's exposure to the evil gods of the sha during the Pandaria's mists which drove his mind completely crazy and gave him that terrifying troll physique. Prior to that, in the Icecrown raid you can see that garrosh was much smaller in size and thrall used to be larger than him. He joined the iron horde, and became stronger with them.

He was a hard worker and became stronger due to his "real iron bros from the gangsta scene".

I think that thrall was hiding for the fight, since he believed he was to blame. He felt that way until he had the conversation in Korthia with his mother.

Garrosh was aware that he had to be killed however, it was only a last resort. Yes he could've gone the warrior way and cut off his head or cleave him with an axe or even brutally strike him or smash his head using the doom hammer but it might have felt too cruel. He ultimately chose to go with the electric chair and executed through a single quick hit of a lightning bolt. It was quite painless to die, I think. He is struck by lightning and immediately goes blind.

What I'm trying tell you is that the warrior within the thrall of him wouldn't be able to kill him. Garrosh's death would have been an even more painful, horrific death. Honor is the right of the warrior. The wild type is shaman. The elements are wilder and untamed. They can be pushed and pulled and will destroy you quickly.

It's possible that thrall lost his warrior skills due to lack of practice since he mostly just uses his shaman powers since the war ended using archimonde in Warcraft 3. It's possible that thrall was weaker as a warrior during this point. With the dragora in his hands, his strength is greater and he could likely take down makgora's day garrosh with an axe toss "for Azeroth!" Like he did to Helya!

Even though Thrall killed Garrosh using his shamanistic skills but he still felt guilty about how he handled the cheap WOW Classic TBC Gold incident, and also how Garrosh ended up due to his decisions as well as the other circumstances that led to his losing his power.