Shuckle GX's Protective Shell Ability states: Prevent all damage done to this Pokémon by attacks from your opponent's Pokémon that have 2 or fewer Energy attached to them.
Suppose I attack Shuckle with a Blacephalon GX.
This Pokemon has three energy cards attached to it.
The Mind Blown attack states: Put any amount of fire energy attached to your Pokémon in the Lost Zone. This attack does 50 damage for each card put in the Lost Zone in this way. I use the attack and put three fire energy into the Lost Zone.
Shuckle GX takes zero damage.
I have just learned that Shuckle GX's ability counts the energy attached to Blacephelon GX after the attack has been made, not before. To knock out Shuckle GX, one must account for having at least three energy attached to your Pokemon after the attack in order for the damage to go through. In other words, Blacephelon GX needs at least five fire energy to take down a Shuckle GX with weakness damage, and seven fire energy to take down a Shuckle GX if weakness is negated (assuming Blacephelon GX has no attack enhancing items or Beast Energy attached to it).
Shuckle GX is one cruel hombre.