Unlike other battles, you can't flee and port back if you die. If so, it will not count. Also, don't pay any attention to your quest arrow. It will still say you need to battle Othin, even though you just won. It changes to the next quest eventually, usually after you leave Othin's area, but sometimes not until you go back to the GH commons and talk to the King.
The frosthaim 'instance' is so confusing because I killed Stormbringer once outside the instance and then again in the instance. No ORBS at all in this instance so you must leave just to have to redue it again from the start! GRRR Thought that you could leave a instance and come back again with in 15 minutes or so. guess KI took that out? Q; where is the true Ivan?