The key wording is "before applying Weakness and Resistance". That indicates the effect is on the Pokemon attacking, and since Combusken is that Pokemon the effect is on him. Therefore it doesn't matter what happens to the Defending Pokemon, and as long as something does not remove the effect from Combusken (Benching for example) he will do 30 more during your next turn.During your next turn, each of Combusken's attacks does 30 more damage to the Defending Pokemon (before applying Weakness and Resistance).