This is one game I had to abuse save/reload to its max. Amdusias is an important stage in the game so, you should have a hard save just before entering the Blood Altar for the first time. I know that's water under the bridge now but I 'm saying that for future. If in doubt, save every time you level up.
As for the situation at hand, all I can suggest is to lower the difficulty level for that particular fight. You cannot advance unless you beat him.