Abilities » Mystic Heart | Attacks » Shell Trap
If my opponent used Turtonator GX's "Shell Trap" attack and I have Magearna EX with the "Mystic Heart" Ability in play, will my Pokemon with Metal energy attached get the 8 damage counters from "Shell Trap" if I attack and do damage to Turtonator GX?
No, it will not receive the 8 damage counters. Mystic Heart prevents all effects of your opponent's attacks, and Shell Trap's damage counters are an effect of your opponent's attack.
Source: TPCi Rules Team (2017-08-03)
Abilities » Mystic Heart | Attacks » Shadow Stitching
If Magearna EX with the "Mystic Heart" Ability is in play, are all Pokemon with Metal Energy attached protected from Greninja's "Shadow Stitching" attack?
If Magearna EX has a Metal Energy attached to it, all Pokemon with Metal Energy attached are protected; however, if Magearna EX does not have a Metal Energy attached to it, its Mystic Heart Ability is shut off and no Pokemon with Metal Energy attached are protected.
Source: TPCi Rules Team (2017-02-02)