He's scripted not to die right away but to say something like: "Oh my god I'm dying"
first. If you break this script (e.g. by fake-talk) he'll never die. Your only chance is to reload and find out what caused the bug then battle him again. If he's in the state you described you can't even cheat him to death and even if you could (Shapeshift->MIndFlayer) you'd probably miss the trigger that takes you back to the ending sequence.