I think making the current Holy usable at range would make it less fun and interesting.

There are essentially no downsides to Gravity, Art of War, and Dyskrasia because either they're usable at-range or have no cast time. Holy, on the other hand, is a commitment to planting yourself near(-ish) to trash mobs, which is its own risk-reward mini-game, especially if the trash has become immune to Stun.

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The comparison to Cure 3 doesn't quite work. Cure 3 always targets a friendly, with the default target being yourself. Holy would become a damage-dealing spell that targets an enemy or a very specific friendly (namely, yourself), and whether it deals damage to the target itself would depend on the target. The tooltip would be a bit clunky.