c’thun druid

Aggro Mulligans

against aggro you want efficient early game cards beckoner of evil and c'thuns desciple are good examples as they let you not only buff c'thun but also to trade efficiently. ferral rage is good way to deal damage to a high threat enemy minion. wrath could be used as a removal and as a card draw mechanic. you still want to keep wildgrowth to be able to play your 6 and 7 mana taunts as soon as possible. if you can stay at quite big amount of life then it might be reasonable to keep twin emperor vek'lor since 2 4/6 taunts will in most cases be sufficient to stop aggro. also do not think about your c'thun combo against aggro decks. it might be a good idea to play brann on turn 6 if your enemy is mostly concentrated on smorking your face to get 3 4/6 taunts turn 7 and basically win game against any facecentric deck

Midrange Mulligans

against midrange you want to play ramp and also buff your c'thun (the later is prioritized) this way you can play your overstatted minions later on. klaxxi and the likes of it will mostly deal pretty nicely with dragons since your minions are better statted it seems (dragons seem to be the only midrange oriented tribe, plus take into consideration that the most viable dragon priest loses it's most important earlygame buff cards which made deck sufficient (velen's chosen) and also remember that they have no realy good ways to deal with emperor vek'lor and dark arakkoa (since they need to save hard removals for c'thun, doomcaller and thaurissan).

Combo Mulligans

 it seems that druids can buff c'thun more consistently than any other class except for rogue. and rogues do not seem to be very consistent as a c'thun based deck. so all in all the only troublesome c'thun deck should be warrior and priest. for priest you have to do manage the damage you deal with your combo since priests can not play c'thun on the same turn as brann, keeping your health relatively high will be enough to defeat them. warriors are an issue. they have a huge array of removals and quite good c'thun sinergy.the only problem they have is, they have to pay a lot of card quality if they want to play brann + c'thun + (possibly) charge. their combo is so difficult' they have to cycle through their whole deck to make it work, and warriors do not have good ways to draw cards in a c'thun based deck. all in all it makes your deck much faster, which means a possibility to overwhelm your opponent with sheer tempo.

Control Mulligans

against control ramp is really good as well as buffing your c'thun. against decks that tend to go into fatigue you might use nourish to gain 2 mana crystals. this way you can play your efficient midrange minions to dominate the board and make your enemy to waste removals. after that your combo should be sufficient to close out the game, since brann + c'thun will deal (at most, without doomcaller being used) 48-50 damage. and since against most control decks you have the board the only possible survivor after combo is warrior (theoretically, but is does not rly seem that warriors will be fast enough to gain 20+ armor by the time you use your combo, since this deck aims to close out the game in at most 15-20 turns).

against freezemage the best thing you can do is play minions and use your heropower every turn. in that case you're more likely to be able to survive the barrage of spells mage is to unleash after playing alex. also save ferral rage as a way to heal for 8.