Nurse Gargantuan (all other, supposedly OP cards) are generally overpowered when you play them against the right decks.
Whenever I see a thread about some card being overpowered, that tells me: "well, there's a solid card I should be using more of!" but I always find that they all have a counter when using the right deck and up to now things are pretty balanced.
Every plant deck should have some sort of hard removal, like squash or a freeze combo. Alternatively, at 7 sun/brain a good aggressive deck should be about to finish off the opponent, and if the opponent used up their whole turn to play one creature, there are a number of cards that can counter it (freezing, destroying, bouncing) while you deal the last few scraps of damage.
My suggestion to anyone finding that there is a particular card that they are having trouble with: if you can, try it out yourself and play against a highly ranked player to see how they deal with it.
If not, sure, come over to the forums but instead of posing it as a hyperbolic game design problem, consider it a strategic question: how do I deal with "this"? It will generate much more constructive answers and prevent trolling. It also helps the game designers get a better idea of how people are using the cards.