Image: Rebel Wolves / Bandai NamcoOfficial screenshot from Rebel Wolves / Bandai Namco.
Master of Arms is earned by buying an Ultimate Perk in the Swordmastery tree. It takes a big pile of perk points, so plan where they go.
Spend 35 perk points in Swordmastery to open the Ultimate tier, then spend 4 more on an Ultimate Perk. The trophy pops on purchase.
Make a manual save before spending. Buy the Ultimate, then reload and put the same points into Witchcraft for Arch Warlock. Swordmastery manuals also help you build this tree.