I tried to engineer a cave-in to lock him in place, but I forgot that the new channeling system makes ramps instead of channels (I HATE this... We can already make ramps with you, know... RAMPS! Why does channeling function identically to ramps now?!? The only difference is the Z-level you designate it on...) and he happily took the ramp up to slay my still-working miners. A slew of dwarfs came to reclaim the miner's bodies, who upon seeing the beast ran away... leading him into my dining room.
He massacred everyone there. I redirected my military there, hoping that even if they couldn't kill him they could hold him off until the rest of my fort evacuated. Not so; he killed my legendary weaponsmith and my high master blacksmith, as well as a few farmers before everyone else ran away. He proceeded to finish off my military, as well as the squad of war elephants I was training up. I forbid the dining room doors to stop anyone from going in, and apparently, unlike HFS Forgotten Beasts don't seek out buildings to destroy. So he sat in my legendary dining room, with half my fortress as kills, doing nothing.
Of course, while I forbid the dining room doors, I forgot to forbid the stuff in the dining room itself... and one of my dwarves entered it from the route that the beast got in there in the first place to collect the bodies. Upon seeing the dwarf, this beast charged straight at him. And into my well. He fell, oh... 5 z-stories, and was split in half by the impact.
Oddly enough, despite losing half my fort's population, nobody is unhappy. It still sucks that I lost several very important dwarfs, but my expedition leader wasn't killed, so I should be able to rebuild. I guess it's a good thing that he didn't destroy my legendary dining room, or else I would have a nice tantrum spiral going on.