By both playing with Greed/Egg Thief/Cedric Sparklesack/Skeleton Key and playing against them, I feel this team is certainly overpowered.
Main reasons are 1) Egg Thief fills up once and never worries about refill ever again, and 2) with 200/200 gold bonus Skeleton key kills everything players owned with only one shot.
I have a suggestion, however not to fix the two reasons I mentioned above, rather change her color to Green/Yellow, so either 1st slot Greed or 1st slot Cedric will slow down Egg Thief a bit, and put her or Key 1st slot will be more risky because this will both mitigate their summon ability and expose them to Skull damage.
This change will theoretically slow down Egg Thief on both defense and offense side, but maintain everything else on current level if this damage output is what Dev intended the whole time.
However, I’m absolutely okay with increasing her mana cost a bit, 12 mana cost destroy the whole board is kinda absurd.