As is, Stone Demon isn't very effective because it has little utility beyond the mana trade (which is generally not favorable). It costs 80 mana to cast, but only 50 to destroy with a quick Spit double tap.
I believe a slight modification to how it works would make it much more useful, fun and interesting to counter:
Instead of letting the Demon be destroyed by attacking it:
1) make it invincible,
2) give it a fixed number of projectiles to fire,
3) have it disappear on its own when it runs out.
Also add some big green glowing gems to the statue's front side equal to the number of projectiles it has left so it's clear to everyone how long it will last.
In this way, it can't be trivially negated by shooting it and the opposing player needs to find a clever way to counter or deal with it (dodging, avoiding, casting Twins, reflecting, absorbing, etc.) Using Stone Demon as a distraction or area denial becomes much more potent because it remains a threat until all of its projectiles have been exhausted.
This frees you up to tune its cost without the mana trade being a factor.