User blog comment:Commander Marko/Commander Marko's set of Custom Patch Notes! (I regret doing this...)/@comment-6061249-20140709191217/@comment-7709681-20140710153520

@ Commander Marko: Normally champions that can both deal percentual damage and reduce defenses (Which are only eight:, , , , , , and ) reduce them by ~25% (With the exception of ).

So yes, I believe is not healthy that amount, and no, just attacking isn't enough. His flat decrease would actually take from squishy targets ~25% of their magic resist and ~12.5% from brusiers and tanks (If focused on increasing magic resist), that is much healthier than taking up to almost two thirds of their magic resist.