I don't know if there is even a fix to this, but please try to help.

Spoiler Alert:I am at the point in the game where I am bringing Auriel's bow to lord Harkon(well, i'm going to kill him) and in my way is an unkillable dragon. Inside his castle, It is on a staircase glitched into the floor, nothing will move it. FusRoDah does nothing dragonrend does nothing. When all his health is gone, he does not die for me to absorb his soul, he still stands in my way. I can't get past it, old saves don't work, he still appears. The instant kill affect of Mehrune Dagons blade does not work either. I don't know what else to do