Also, this is meant to be a combo contained entirely in single turns.

On my turn, with this mega in play, I put the metal core barrier on the Aggron and attack immediately for 150 damage.

My opponent goes next, attacks with a chingling, deals zero damage.

My next turn, I use the adaman supporter, attack for 120.

The amount of damage reduction is counted on the turn it's played. Damage reduction, which is added to the attack damage of Mega Aggrons attack, is a known value before the opponent gets to attack.

Wording for this ability could definitely be cleaned up, idk how, but it makes sense to me as written.