you cant assume that all attacks will be successful, otherwise there would be no "upon successful attack" cards in the game, they would just be "this happens every turn" cards. and even if it is a successful hit despite a shield reducing the damage, the fact that it is split between a spark and a battlestar would double the damage reduced by the shield, so if it were a -2 shield, the battlestar would do 1 damage and so would the spark. for the upped, if it was a -3 shield the galactic battlestar would do 3 damage and the ball lightning would do 2 - twice as much damage reduction than would be done if it were simplified to one creature as you want to do.