Fire Queen and Ferox are already sort of a joke. The only reason to lose to them is on a bad draw where you can't get a Dim Shield out before they kill you or something along those lines. If you get your ability to stall going with a few towers, you're going to win. I guess I can imagine a situation where this is a complete game-changer against FQ, but so is an early quinted Oty or access to Fire Storm/Bone Wall at the same time. That's 5 Quantum(+1 a turn), or 10 quantum (distributed among two elements) in a rainbow for the same effect. Rainbows already need death quantum for Bone Wall (necessary to protect your permanents/late game defense) and Graveyard (to ensure that you have something to mutate if you have to eat your own fireflies). I feel like this card is alright, but I think it's more than a stretch to say it's overpowered. It's really situational and fairly expensive as well, you have to be sure they need their healing the following turn and there's nothing they can do to delay if you really want it to be a game-changer.
The thing I'm not sure we're realizing is that it will NEVER actually do 99 damage. It will, however, prevent a player/CPU from playing Miracle, meaning you have a chance to kill them. This card can be countered by the opponent playing a sundial the same turn it is played. This means your creatures won't attack, and they can use their Miracle the following turn.
I like the card, except I feel like SoG should be immune to its effects. I think it's alright to have it effect EB, although it's a similar effect, because it takes so much work to actually get more than a few EBs out.
