Phase One: Normal Dettlaff
When the fight begins, you’ll be dealing with Dettlaff in his most basic vampiric form. He’ll still look mostly human, except for the creepy fingers and bat-like face, making him easier to read. He’ll rely purely on his speed and strength to bring you down, making Yrden and Quen much more useful during this phase than any of the others. His attacks all involve him using his smoke form to rush in and start a melee combo.
The easiest way to deal with Dettlaff in this form is to wait for him to rush you. He blocks most incoming attacks with his claws, so trying to go on the offensive is usually a waste of time. Instead, let him come to you and dodge to his side once he appears. This will create an opportunity for you to wail away at his side before he rushes away, only to repeat the process.
It’s pretty straightforward as long as you don’t grow impatient. His ability to counter you at the most inopportune moments is deadly, so never bite off more than you can chew.
After a few back and forth sessions, you should be able to take off about a third of Dettlaff’s health, triggering the second phase. The Beast of Beauclair will grow enraged, transforming into a more frightening form.