For my party, this was last night in the Cloakwood Mines. They had gotten pounded by that Hareishan one level up (forcing a reload, as my MC, Eilrie, was killed). Of course, that was due to a massive tactical failure on Isra's part that resulted in guards pouring out from too many directions, but still.
So, expecting Davaeorn to be, well, hard - especially with SCS installed - I had the party go back to High Hedge for some potions of explosions and spells. Aura spent an afternoon bulking up her spell book. I optimized Tenya's spells, switching out healing for skeleton warriors. They were ready to rumble.
They get to the bottom of the mine/enclave and Imoen is clearing traps when he (somehow) sees her and casts a skull trap at her. I'm not sure how, as she was still invisible and I didn't see anything about spells that allow him to see through that. (It's possible she set off the last trap, but I'm almost positive she cleared it just before he spoke and she took damage) Weirder, he didn't continue the attack - he just stood waiting.
Psyched up, they retreat to one of his ante areas where the dead guard was and form a plan. Imoen sneaks around behind him for the backstab while Tenya's lesser water elemental attacks from the front. I expect to have to get Immy out of there fast, so I have Isra and Sirene ready at each corner of a hallway.
Go time. Because they are still invisible, the elemental attacks first and scores a hit. Almost immediately, Imoen rolls a natural 20 and Davaeorn is laying dead on the floor. I'm still shocked when the chapter narration starts. He didn't even get a chance to start casting! Now the party has a bunch of potions of spell blocking and explosions left over...
The only mistake was forgetting to loot the one dark corner with the jelly. Isra saw it, but they totally neglected to kill the thing and examine the cupboards. The whole place is flooded now, so no going back. Still, that was unbelievable.