For a planeswalker, excess damage means damage in excess of the planeswalker's loyalty (the number of loyalty counters it has on it). For a battle, excess damage means damage in excess of the battle's defense (the number of defense counters it has on it).

Excess damage has been dealt to a creature if the damage dealt to it is greater than lethal damage. Usually, this means damage greater than its toughness, although damage already marked on the creature is taken into account.

Improvise doesn't change Bottle-Cap Blast's mana cost or mana value, and it can't reduce Bottle-Cap Blast's cost below .