Unless noted otherwise, damage is calculated and applied before any other effects of attacks, including discarding Energy. I'm assuming you are asking in regard to whether Strong Energy's damage boost applies when using All cells Burn. The answer is yes, since Strong Energy is not discarded until after the damage is applied. So, if a Zygarde-EX had one Strong Energy attached, the attack would do 220 damage. You are also not required to specifically discard Strong Energy. If a Zygarde-EX has more than three Energy attached, you may choose which ones are dacarded.