just follow him around with a HN unit until he goes into a city, and then kill him. He's useless outside of cities for the most part anyway so he'll have to go into one at some point.
Also, first strikes do seem to hurt him. Not sure if they can kill him though. Collateral damage would be a good way of weakening him so your first strikes could have a chance.
Does he cause you a problem? When I played Balseraphs he is only a moderate boost at most, and he does die quickly if I dont protect him.