How would you change the description while keeping it simple? The word “summon” indicates that it is not part of “you” (it is a summoned thing) and the description also tells the reader that it is the one taking mana not the attack/projectile. You are right. It is your manashield taking your MP, not you taking damage and losing MP instead of HP. I still don’t understand why you think a summoned shield is part of your “body”.
The description for damage/damage reduction has nothing to do with how manashield works. The Alchemy mythic clearly tells you that you lose MP instead of HP depending on which is higher when YOU take damage.
What can we say to make it clearer that the manashield and you are separate entities?
Edit:
Damage calculation in almost every game with projectiles are calculated in separate parts (when the projectile is launched and when it hits something), this one is no exception.