So, I finished up Mooshu at level 45 and returned to Merle Ambrose, because that's what was next. While unlocking the quest to open Dragon Spire, I also got a quest from him to head back to Grizzleheim. However, after following that quest to the point where I talk with Bjorn Ironclaws, there is no follow up quest.
I'm confused. I unlocked the quest to fight Jotun at level 40 (and I'd seen Merle a couple of times since then with no new quest). Do I have to finish that one (stopped because I didn't feel like trying to solo three 8000-9000 rank 7 bosses at once. yes, I saw that if you clear the other two instances the brothers won't show up, but I didn't realize that then) before this one (presumably to Ravenscar) continues?
You have to defeat Jotun before you can continue on and yes, the next quest will lead you to Ravenscar. About fighting three 8000-9000 bosses, you have two choices....either defeat the brothers before you go see Jotun or bring some help. I had two Grand Master wizards, myself included, and two wizards about level 30 in there with me and the battle still took almost 45 min. We didn't take the other two out before entering, like you I didn't want to go through it all. Good luck with your questing and I hope this helps some!