Grieg's End  

The mob you are thinking of was the "Servitor of Luclin." We didn't even attempt him as we only had one cleric.. and just like you, our tests showed (tests, meaning, I tested his melee damage, and a shaman sacrificed :p to see if it was slowable). Flurries, immune to slow, max hit on me was 348 (I'm a monk with roughly 1200AC) Red to me at 60.

Also, that key will work on another locked door. If you go to that main corridor (the HUGE one) go to the left side, instead of the right (right side leads to Servitor). It will bring you to a one way portal. Stepping on it will put you in a room with 2 mobs. If you go past that room and up a little set of stairs, you'll find Grieg.

Heres a couple links to some screenshots I took tonight while we were there:

The Servitor:
http://www.afflictionguild.com/media/users/Jaiieq/Servitor.jpg

Greig (Hes flagged as a vendor o_O):
http://www.afflictionguild.com/media/users/Jaiieq/Grieg.jpg

And a sample of Griegs melee output (I didn't last long due to the 4 level 60 SK's that guard him, which you'll notice one of the 1000 point HTs on the screenshot):
http://www.afflictionguild.com/media/users/Jaiieq/Griegmelee.jpg

There is only one area we haven't explored yet in Greigs that I plan on hitting next time I go.