Neither Ambrose nor Lucius were currently at full strength, Lucius had not had chance to perform a human sacrifice thanks to Evie whilst Ambrose had given up fresh blood. Vampires are vicious predators and extremely difficult to kill or even harm plus they have speed and strength on their side. Witches on the other hand are mortal, essentially humans with added magic and dramatics. Despite this Lucius was confident that he could come out on top, magic should never be underestimated. Especially not by a Vampire.
Rule 1 of Combat. Note your environment Buffy and Evie were still in the way, this fight was somewhat overdue anyway. Both Supernaturals had been wanting it since they first met, the girls couldn't stop it and it would only continue to get worse should they restrain themselves. Buffy wouldn't go upstairs of her own free-will, she was far too stubborn. The solution then was to remove her from the situation against her will.
Lucius glanced over at Evie, shooting a slight smile at her "Catch". He snapped his fingers and instantly rendered Buffy unconscious, although the Slayer was protected against many Supernatural Powers - Magic was not one of them. Quite deliberately in fact.
"Evienne, take Buffy upstairs, lay her down on a bed and look after her. She's going to need all her strength when this is over" Lucius ordered, the tone of his voice stating clearly that this was not up for debate.
A lifetime of disciplined combat training meant that Lucius subconsciously shifted his weight to gain a better center of gravity. Rule 2. A good Defense is the basis of Offense His body would not be making physical attacks, not against a Vampire. He had magic as his offense.