If it's "just a reminder", then they shouldn't take double from earth auras. Either the rulebook notations need to be stricken out, or the monster card should read "Takes double from Earth carrier", imo. Telling a new player "Yes, double from Healing means you take double from Earth auras, but if it's a healing spell, just take double the amount it would heal, not double the amount of damage that the rulebook says you take" is just confusing. And when you get a monster card that says "Undead", but doesn't say "Takes double from healing", you're having to tell the new player "I know it says Undead take 60 pts from CMW, but really, you only take 30. No, it's not on the card, just trust me."
Flamebolt doesn't say "does 40 pts of damage to ice elementals". Why say "CMW does 60 pts to undead" if we're going to put "takes double damage from Healing" on the card?