If it turns out that Damian's demigodhood is powered by a magical ring that he lost somewhere, then by all means, call in a halfling.
But as far as I know, his demigodness is powered by the soul of the Lord of Chaos, and I think that not even the sword Narsil is sharp enough to cut the soul out of a demigod.
Different ending sequences are certainly possible to do, but this series has never done that before. I think there's too much blood and hate for a Damian redemption story to be believable.
Even if Damian could be weakened enough that a non-demigod could kill him, my point is that the emotional heart of the conflict is not between Damian and Bozo The Random New Protagonist, It's between Damian and his foster father Lucian. Lucian's beheading of Ygerna was the act that Damian could not forgive, the act that turned him to the darkness.
Just look at the scene in Divinity 2 where Damian and Zandalor show up in the Battle Tower after the defeat of Laiken. Damian doesn't give the Dragon Knight a second glance - he doesn't care one way or the other. It's Zandalor, the friend of Lucian who draws Damian's ire.
Nothing Bozo TRNP has done could compare with that.
