I did the Greybough quest in wild, it was hard! I used a ramp deck (Breath of Dreams, Wild Growth) and a dragons package (Big Ol Whelp, Azure Drake, Emerald Explorer) and plenty of draw cards (Breath of Dreams, Branching Paths) with 1 Witchwood Piper and Lorekeeper Polkeit to reliably draw Greybough.
Your combo is this: Greybough + Lightning Bloom + Germination. Try to get 2 or 3 Greyboughs on the board, and then attack them into enemy minions over and over. Do NOT attack the opponent hero. Otherwise if you are winning they will concede.
If the opponent plays a board clear, use Hadronox or N'Zoth to get the greyboughs back. There is also a new 4 mana card that spawns 2 whelps, they can copy the effect.
Although it's hard, this deck is fun. If you ramp early and well, play some dragons, and draw into Greybough, you should be able to spawn around 5 greybough per game. Once in a while you will play a game that goes long - one game I spawned over 50 greybough, but you need an opponent that is good enough to cooperative with you, keep playing and not concede.
Priest "silence and destroy all minions" and the new mage card "Poof! All minions are gone" can ruin this deck but otherwise you can grind greyboughs quite consistently.
This was the hard quest Ive done so far (Ive completed all quests 4 classes) but it's doable - good luck and have fun!