I think the main issue with this card is that it has a lot of potential but is underused because there are so many alternatives.
My fix would be to do "Heal target creature to its max HP. If target creature is
or
, kill them instead." with the same cost.
That way some of the grav dragons can be healed to full HP, a creature with a ton of HP buffs that you don't want killed can be healed with that, it removes the ability to heal self because Luciferin/Luciferase does that already, some insanely buffed dark/death creatures can be instagibbed.
Although I really think the issue with Holy Flash is that it just doesn't have a place to properly work. It's only a heal. Lucis can heal AND fuel Hope, Blessing adds attack and defense, Miracle grants full HP minus one, Life has a 20 heal for just a couple extra quanta.
So really instead of buffing/nerfing the card to make it fit in, build something USING the card itself and show everyone how great of a card it can be.