(I'm limiting spoilers here to the House of Hope. I'm still in my first playthrough, so I haven't seen it all yet.)

We were warned more than a few times that it was a fool's errand, but when my party learned the Orphic Hammer was at Raphael's place, and acquired the means to get there, the prospect of a heist was irresistable.

Of course, Raphael showed up just as we were about to leave, and he wanted blood. It was the toughest fight we've had so far. Here's what worked:

Party Members: (All level 12. AC range 17-20. Companions as their default classes.)

• My College of Lore bard
• Shadowheart
• Karlach
• Gale

Followers & significant summons:

• Conjure Elemental: Air Myrmidon
• Conjure Elemental: Earth Myrmidon
• Create Undead: Ancient Servant (mummy)
• Hope

Especially notable tools:

• Envoy's Amulet (bard)
• Sussur Greatsword (Karlach)
• Staff of Cherished Necromancy (Gale)
• Fire resistance elixirs
• Maybe 15-20 healing potions (mostly greater, some superior and regular)

Preparation:

• Fire resistance elixirs for everyone. (They were already active from earlier fights.)
• Defeat all enemies in the house before trying to leave, so they couldn't wander in and make the final fight harder. (Some of their corpses were useful in the next step...)
• Summon the mightiest summons we could that weren't made of fire, water, or ice.
• Drink deeply from the boudoir faucets, to replenish everyone's resources.
• Persuade Yurgir the orthon to turn against Raphael and join our side. This was a DC 30 skill check IIRC, so probably not available to every party. Fortunately for us, my bard is a persuasion expert, was fully inspired, and had the Envoy's Amulet and Shadowheart's Guidance. We talked him into it after a re-roll or two.
• Enjoy the battle song.

Battle:

• Cast Planar Binding on Raphael immediately. Maybe twice. He shakes it off quickly (I think it's just one turn) but it keeps him and some of his minions busy just long enough for us to have a chance at retreating.
• Leave the foyer ASAP. With all those fiends on raised platforms and moving to surround us, there was just too much heat in that room. Let our summons tank for us as we cast some debuffs and run for the door, and then have them follow us out.
• Regroup at the far end of the passage to the feast hall. This let us choose our positioning in a meaningful way, and provided a choke point for the enemies.
• Engage the fiends one or two at a time as they trickle through the door. Gale's staff-boosted necrotic attacks work well. AoE damage and debuffs (like the air myrmidon's Raging Vortex) keep them from being too threatening. Meanwhile, let Yurgir work on them from behind.
• Heal mostly with potions, since quaffing only consumes bonus actions, leaving spell slots and actions for enemies. Toss potions at the feet of party members in need before their next turn, and whenever two or more are clustered closely enough to hit with a single potion. Heal Yurgir with spells when possible, since he doesn't spend much time in potion range.
• Continue until Raphael is the last enemy standing.

Master of the house:

• Hit Raphael with all the debuffs we can; especially Silence. Karlach does great work with the Sussur Greatsword, and the myrmidon's Raging Vortex seems to work well. This keeps him (somewhat) less dangerous long enough for the next step.
• Send the summons past Raphael, through the door, and back into the foyer to destroy the pillars. Their teleportation abilities are great for this. It really pissed off Raphael, and he killed them all, but not before they could finish the job.
• Summon new summons. The more the merrier! Quothe the raven and Scratch were with us, though not much use beyond a couple brief distractions.
• Lure Raphael close to a relatively open area, where the party can take aim at him without bunching up too much. (His AoE combos can be tough to heal through if they hit more than two targets each.)
• Resume the debuffs, and pour into him every damage source available that he can't nullify or retaliate against. (Avoid radiant damage.) Necrotic and force damage work well.

I was skeptical more than once that we could win... but somehow, we did. Whew.

Yurgir had so much fun that he offered to join us later in the campaign. I guess bard school pays off. :)

Anyone else have a strategy that turned out brilliant or failed hilariously?