If there's one biggest difference between Aster's time and the current generation, that is the absence of standard games. Of course, it's not like it completely disappeared from circulation, but only a few have time to spare playing games.
As such, the "standard" procedures for hunting certain monsters have been lost, and treated as "arcane knowledge."
The use of healing on undead monsters to damage them was one of those. Of course, using holy element attacks was a commonly discovered trait, but nobody would even think of healing the enemy, after all. As such, it became a forgotten knowledge lost due to the long passage of time.
"Haaap!"
Now that the eight mummies were hit by the healing spell, their bodies became sluggish, almost losing half of their mobility. Furthermore, their defenses became literally paper-like, making it easy for everyone else to damage them.
"Holy Smite!"
"Void Palm!"
"Twin Fang!"
