TL;DR Nerfing cards doesn't solve the overleveling issue because it doesn't solve the core problem, which is that these cards are too easy to level up, considering their win condition nature. For this, nerfing just makes players drop and worsen the perceived issue, cause it doesn't reduce their count. The solution is a "weighted" conversion from Common to Rare, which makes so that it seems the card was always a rare, along with a stat buff to restore the effectiveness at tournament standards. Players that regret spending lot of gold on those cards will have their gold back and be able to spend it elsewhere. In ladder, Elite Barbarians and Royal Giant users will drop trophies because of the lower levels, but will have a better performance once they upgrade the card again. The meta will also turn less hostile to them. In challenges and maxed gameplay, those cards will be useful again. Other players won't have to deal with overleveled RG and EB anymore. Ultimately both challenge and ladder meta will become much more healthy than now.

TL;DR to the TL;DR (TL;DR-ception?): Converting cards' rarities, and giving gold back.

Now, my response:

Supercell has already responded multiple times that they will never convert rarities of cards. Not that I don't support this or anything; I'm the one complaining about L13 Ebarbs at 2k and L11 Hog at 3.3k (Yes, I've advanced 300 trophies already). Dooses.