Lightshow is overcosted due to the random targeting. While the spell can theoretically go infinite a 3 mana spell for 2 damage twice to start with is not nearly powerful enough to compete in this meta when 1 drops can have 3 health.
In short Lightshow should cost 2 mana much like the spell first flame. And while there is a mana reduction minion, once again the issue is getting it online.
Currently the better spell option for a combo turn is arcane bolt since mage is mostly concerned with survivability using solid alibi.