So, my tentative suggestion for a plan would be to try to enter via the upper floors (assuming Mog is on the first floor), that way we can probably take out several of the thralls while they're (relatively) isolated. Even if Mog has most of them waiting on him, it's likely that some would be 'off-duty' elsewhere in the house. If we're really lucky, we might even be able to maintain stealth while doing so.
Brand is a grappler and a tank; he should at the very least be able to keep Mog occupied, and maybe even lock him down completely, especially if Brand gets a free clinch in due to Mateusz using Serpent's Eye on the giant.
EDIT: Should also add that Brandon, with his high strength and Holy Bound knack, can easily ferry people up to the roof (he's got a 20 yard vertical leap; easily enough to clear 3 stories). Along with Julian's rope and other equipment, that should let us enter and navigate the precarious 3rd floor.
The neighbors will probably notice and call the cops on us, but in this part of town, their response time has gotta be pretty low, and that'll drop further if they suspect it's a prank call (for instance, if the caller claims that someone is jumping to and from a 3rd story roof). Should be plenty of time to fight and retrieve the dagger, though getting away might be tight.