In my opinion, the stones should be able also to destroy weapons and shields as long as they aren't invulnerable. Just think of an opponent with 4 pillars and a weapon... the chance to target the weapon is already only 20%, decreased by the fact that the stones can fail without hitting anything. Thinking about the huge cost of the Levitator, it shouldn't be OP at all. All in all, I really like the mechanic as it is now.
Minor thing: the description of the upped stone says "Sacrifice Levitating Stone", but it's name is "Weightless Stone".