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)

Relevant cards

Note: All applicable cards may not be listed below.
SM2_131
SM2_148
SM2_18
SMA_SV52
SMA_SV52
XY11_110
XY11_75