Even at 3-4 HP this card starts being really... not very useful, as the chance gets reduced to below coinflip.
100% - 10%*HP, maybe?
Agreed, this card has limited utility on high hp creatures--but would be significantly better than the current
spell (Thunderstorm.) Also, remember this could be used in conjunction with sniping ability of Owl's Eye. The benefit is when you play this on large crowds, not single creatures. Use sniping to hit single creatures with certainty. I'm trying to make a spell card that improves
but don't want to stray too far or it'll be OP.
Consider the scenario #1: 4 dragons with 6 hp each.
My proposal would give 17% chance to remove any one of them (83% to miss.) So the chance of missing all 4 is 83% x 83% x 83% x 83% = 47%. That's leaves 53% chance of getting at least one of them (possibly more.) Not too shabby.
Your proposal would give a 40% chance of removing any one of them (60% to miss.) So the chance of missing all 4 is 60% x 60% x 60% x 60% = 13%. That's 87% of getting at least one of them (possibly more.) That's pretty powerful stuff, and probably OP for a single spell card.
Consider scenario 2: 4 dragons with 10 hp each.
My proposal would give 10% chance of removing any of them (90% chance to miss.) So the chance of missing all 4 is ~66%, meaning 33% chance to get at least one of them.
Your proposal would give 0% chance of removing any of them. That leaves 100% of missing all 4.
Which method is better? That's a personal preference, but I prefer some chance to remove the big fellas to no chance at all.