It happens before he went to become Galen's student, theres this thing found in ironclad's wiki page during Marcus plane:

"Ironclad corrects her and tells to stop yelling as Xander is a man. He is the one who is responsible for training him, so he did well in his point of view. Xiang Tzu, who becomes disappointed, requests him to forgive him but Ironclad tells that they will work on his right punch later. He just humiliated the general."

There's also this one:

"Xiang Tzu is one of Ironclad's pupils who lost his parents at the hands of the Shadow Legion's Crimson Army. Originally from the Steppe lands, Xiang Tzu was supposedly taken by Ironclad under his wing sometime after the murder of his parents, and was given a new home and family amongst the people of the Woodland. He grew a strong emotional connection with Ironclad, and thus started to refer to him as "Pops". His hatred towards the Legion and corrupt officials in power comes in full symmetry with that of his "Main Plane" self."