Can you defeat Brencis during the prologue?
Short answer: The first Brencis phase can be defeated with strong Perfect Blocks and dodges, but the following Grotesque Ascendant phase is an extreme challenge and is not a reliable way to finish the prologue.
What the sources confirm
In Bad Blood, the intended path is to lose so the story continues. A skilled player can win the regular Brencis encounter, revealing his true form and awarding the early Realmwalker trophy according to the retail walkthroughs. The Ascendant phase has multiple health phases, deals lethal damage, and cannot be quicksaved between attempts.
Treat the win as an optional challenge, not a required objective. Make a manual save before the fight and lower difficulty only if you are deliberately testing it.
Timing: Bad Blood's published costs are one segment to search the mine, one to aid Ludwig, two to return to Laslea (reduced to one after saving Vlad), and one for the optional Vlad objective.
Sources: PC Gamer Brencis prologue guide and PowerPyx Bad Blood walkthrough.