Are you on PC?
'Resurrect' the corpse through the Console and then kill the thing again, see if the problem is persistent
I could not select the corpse to target it on console. I went to the extent of reinstalling everything.
Anyway I am lucky enough to find an autosave at the start of that particular level. I tried replaying it, so far no problems. I hope it will not happen when I reach the boss again.
I hope this can help remind you guys to save often and systematically.
For the record, despite this bug I am very impressed with this game so far. All Bethesda games I played had WAY worse bugs. Before Skyrim was released, I told my friends "Its a Bethesda game. You ought to wait a year for all patches and DLCs to be released before buying it." Glad I did not wait.