Play Sorcerers strike, get 75 gems plus production.
Play Restoration, spend 8 gems, gain production, receive Sorcerers strike.
Play Sorcerers strike, reduce enemy's tower by some value greater than 90, likely more than 100.
To better balance it, I would say it should cost at least 60 gems and then reduce tower by the amount of remaining gems, not 2x, and not have a gem-gaining option. Since it doesn't require having other gem-costing cards on hand (Mage, Legend, Undead), gathering so many gems in any other keyword deck shouldn't be terribly difficult.
If not a higher cost, it needs a maximum amount of damage that it can deal. The fact that every turn of accumulation makes it reduce the tower by a greater amount is what makes it out of line against existing cards. Only Battalion of fear comes close, and that card -consumes- those recruits while causing attack rather than tower reduction.