Ruling Magmortar(?) vs Ampharos Prime(HGSS)

[Shoyru1444]

Setup:

Opponent's Pokemon: Magmortar (?) (One with the body that heals for 20 damage whenever you attach a fire energy) active, one damage counter on it.

Me: 1 Amphy Prime out.

My opponent attaches an energy to Magmortar. What would happen first; Ampharos hurting Magmortar for ten, then Magmortar healing himself completely, or Magmortar healing himself first, then Ampharos doing 10 damage?

 

[banuffin]

In this scenario, the user controlling Magmortar decides the order in which these events take place (i.e. if the damage from Ampharos' body takes place first or if healing from Magmortar's body takes place first).