Image: Rebel Wolves / Bandai NamcoOfficial screenshot from Rebel Wolves / Bandai Namco.
Ambrus is the youngest of Brencis's three Boyars and the court boss most guides tell you to fight first. He is a sword duellist, and a little prep before the duel makes him far easier.
Disrupt Ambrus's blood-collecting operations around Maragir until his anger meter is high. That unlocks The Gilded Gauntlet and a direct duel. Before it, Iulian plans to poison the blood Ambrus drinks. When Ambrus offers you a drink, tell him a peasant should not drink before a boyar. He drinks first and gets poisoned. Pick any other line and you lose that edge.
His quick slash chain is slow to move with, so step back out of it. For his crouch-and-lunge charge, dodge to the side or counter it, since backing away does not cover the distance. In his shadow strikes the screen darkens and he reappears from several angles. Listen for his shout before each hit and counter on the sound rather than chasing him with your eyes.
You choose what to do with him once he is down. Draining his blood unlocks Scarlet Shield in the Vampirism tree. A phial of his blood from other quests also grants it, so another choice does not lock you out.