I got completely pasted multiple times by Sheliak while taking less than half its hit points in return. had to retreat to my respite to improve gear and make more healing salve and potions. But when I returned to the well, Sheliak did not appear. I've tried going back to the Crossways, re-taking the buff from the observatory, completely closing the game and restarting, but the boss will not reappear,
I had the exact same problem (made a report and everything), but I went in and destroyed the pillars with my pick and Sheliak respawned. It then beat the crap out of me and I died but the initial problem was solved.