Prime Suspect

I've opted to include this picture instead of the actual end fight, since this is by far the hardest fight in the estate (and indeed one of the hardest fights in the act so far).

The Shades and the Rage Demon at the entrance can be dealt with easily if you run up the stair and deal with the trash Shades in the next room first, then pull the ones in the entrance area in there and deal with them one or two at a time.
For the rest of the house you can kite everything, and Gascard himself will die to a Mark + Assassinate combo (and if not, the first arrow that lands after that). But this one fight is something else.

You have a very small area to operate in and a lot of Shades (a second wave will spawn quickly as well). Gascard must be dealt with first otherwise he'll start using Blood Magic. I just used vendetta (save you Assassinates for the Shades!), which took care of him, then I summoned the mabari, used evade to loose the shades I had on me and managed to kill a couple of Shades before my dog died.
At this point the second wave spawned... I stealthed to catch a breather and wait for my Mabari cooldown. As stealth duration was about to run out, I attacked one shade, and Assassinated another, then began running in circles to try and lump the shades together (same as depicted on the screenshot) so I could catch all of them in a combustion grenade. I managed to get one before they broke the stun. Then I just started running again, using Assassinate and dog as soon as they were off cooldown.
It's a very messy fight and I wouldn't suggest doing it without a combustion grenade or two.