Damage/muscle band when base is reduced to zero by an effect

[FunnyBear]

When Mewtwo attacks with Photon Wave, which reduces damage by 30 next turn and Seismitoad Ex with Muscle Band plays Quaking Punch, does QP do zero or 20? I seem to recall something about when base damage is reduced to zero, MB cannot add, but I'm not sure about the order of the damage modifiers.

As this is being researched, please also tell me what a Shaymin EX with Muscle Band does with Sky Return to an Entei (AOS 15) that has used Flame Screen on its previous turn. Zero or twenty? Same issue.

 

[PokePop]

The order of actions is critical here, because once the damage hits zero, you stop calculating.

In this case, the order is:

1. Base damage: 30
2. Effects that happen before Weakness and Resistance. In this case, both effects take place before W&R, per the text on the cards.
Since they both take place before W&R, the owner gets to choose the order. I'll assume they choose to add 20 for the Band first and then reduce the damage by 30 for Photon Wave.

So the end result is 20 damage.

Your second example is a bit different in calculation order, since the effect of Flame Screen is on Entei, and so it is calculated after Weakness and Resistance.

So Shaymin EX would do 30+20, then -30 for an end result of 20.

Same result in this case.