Instead of a heal beam, why not something that does the complete opposite?
It drains the life of an opponent, whilst healing yourself, and reducing their damage whilst adding to your own. No idea on numbers, but it would make a close combat variant much more viable, without being op. When you link the beam the drain begins. If you or your opponent leave the range of the beam, the life drain stops, but the damage reduction/increase remains for a few seconds. No need for the suggested shotgun variant with that ability.
Or how about making the sunbeam vanquishes unrevivable? A small change like that would make quite a difference.