Same case than Donphan's Wreck. You deal the damage beforehand and then complete all other actions. So you first deal 100, then the damage is boosted to 200, attack hits Safeguard Pokemon and THEN the Stadium is discarded. This order is because the way the attack is worded. Saying something like in Skarmory-EX's Joust, "Discard all tools attached before doing damage" in this case (but with a Stadium), would cause the Stadium to be gone before the attack hits.