Mulligans
General Mulligans
you should always keep one of these in your starting hand.
Sure, having 2 embiggens at the start seems nice, but I've gotten it earlier and because it makes my minions very expensive and destroys my start I usually get killed early game if I do that. So the best thing to do is probably to keep only one of these at the start.
Combo Mulligans
you can keep 2 of these in your starting hand, if you already have a dragon in your hand, or replacing 2 other cards in your starting hand, if you're feeling lucky. otherwise, keep one and hope for a dragon draw.
Currently my win rate with this deck is 55% (more than 100 games played). This deck, (as most decks) requires a little bit of luck to draw those embiggens early, not to get crystal stags / oasis surgers from the quest and to not get big cards only on early game (it happened for me a few times and its really frustrating, tho rare), but overall this deck does quite well for its cost and class in this expansion. Basically, if you like playing druid, but got bored with the spell druid decks, you can try this one.