Yeah that's the problem really, it blasts creep waves more often then demigods as far as I can see, and by the time you buy a finger of god you'd probably prefer getting some xp/gold for killing them.

I think, alternatively, the Finger of God's rate of fire should be lowered.
Now, when the Forces of Darkness's FoG strikes, an in game version of Ozzy Osbourne is summoned at that location, cueing the classic 'Smoke on the Water' riff to play once, while he gives the randomly targeted Demigod the finger - who then explodes into Bats and is killed instantly.
When the Forces of Light's FoG strikes, it summons an in game version of Frogboy - appearing out of a swarm of bees, accompanied by their sound - who smacks down the randomly targeted Demigod with his BanHammer who then explodes into small frogs who all hop away, and is also killed instantly.

I believe this would suffice?

The question is not "Is that a good upgrade?" It's "Does God's finger really do that?"