Make magic cancellation not work with a fixed rate versus all types of attacks it works on throughout the entire game. Instead, scale a monster’s ability to penetrate your magic cancellation based on its level, base level, maximum HP, or hit dice.

Hit dice and base level are nice if it’s desirable for all monsters of the same species to have the same MC penetration, but hit dice and maximum HP are a little weird since they aren’t meant as offensive stats. Maximum HP and actual monster level are nice because they can be discovered with a stethoscope.