Enter the Matrix

This is from irish77's FAQ. Hopefully it helps.

Mission: The Dungeon

After an "interesting" encounter with Persephone, you will find
yourself in the Dungeon of the chateau. This level bares a stark
resemblance to what I would image Martha Stewart's basement would look
like. Follow the path open to you until you reach an old wine cellar.
The gate that comes crashing down behind you does well to letting you
know that you are expected. Two vampires will emerge from the far end
of the cellar and try to stop your progress. Remind them why you are
called "The biggest, most badest, MOFO on the planet." Well, they don't
call you that yet, but give them a beating so bad that they will start
to. Once these drones are dealt with, take a second to refill your
focus and life bar before jumping down into the pit of the next room.

This next room is where The Merovingian puts on his doll shows. Either
that or he hosts his fights to the death. It is one of those, I forget.
From the looks of your opponent in the center of the pit, it would
appear that the doll show theory is out. The name of your new friend is
Cujo. Just like the dog that liked the taste of human flesh, this Cujo
is equally evil. Take an all out approach to this one. Throw on your
focus and don't stop attacking. Cujo has better strength and speed then
just about any opponent you've faced so far (expect of course if you
avoided my advice and took on an Agent early on in the game). Just
continue to be relentless and don't stop even if your focus meter gives
out. Eventually Cujo will perform a counter attack that won't hurt you
but it will give him enough time to gain the high ground. After Cujo
has performed this move, he will jump up onto the upper edge of the pit
and fire at you with a shotgun. Avoid his aim by moving closer to the
edge of the pit he is closest to.

Cujo will now sick his dogs on you. Not literally, but you will have to
fight three regular goons before Cujo will hop back into the fray.
Remember to keep your back to the wall nearest to Cujo. If you venture
out into the middle of the pit he will fire at you with the shotgun.
Let the three vampires come to you and dispose of them quickly. Once
Cujo jumps back down into the pit, continue your relentless assault on
him. Eventually you will drain his life away to zero. Just like any
other vampire though, Cujo must be taken out with a steak to the heart.
You can use the crossbow to aid in the process or you can perform a
simple leg sweep to set you up for the dead. Once he is done for, climb
out of the pit and exit through the hole he entered from.