With default AI I noticed the same issue. Someone suggested making a deck rule file (excel files in the DeckRuleSet folder), I tried just setting a "gain for cast" on CTHUN card, but it still would not cast CTHUN. Finally I set a rule for each card that would not allow any card to be played if we can play CTHUN instead and that worked, but its a lot of rules, basically for every card in the deck you say CAST IF, I have less than 10 crystals, or if I have 10 crystals and no CTHUN.
C'Thun 5000 GAIN FOR CAST *
Wrath CAST IF my total crystals=10 my hand has no card named $(C'Thun)
Wrath CAST IF my total crystals<10
Beckoner of Evil CAST IF my total crystals=10 my hand has no card named $(C'Thun)
Beckoner of Evil CAST IF my total crystals<10
Darnassus Aspirant CAST IF my total crystals=10 my hand has no card named $(C'Thun)
Darnassus Aspirant CAST IF my total crystals<10
Loot Hoarder CAST IF my total crystals=10 my hand has no card named $(C'Thun)
Loot Hoarder CAST IF my total crystals<10
Feral Rage CAST IF my total crystals=10 my hand has no card named $(C'Thun)
Feral Rage CAST IF my total crystals<10
Disciple of C'Thun CAST IF my total crystals=10 my hand has no card named $(C'Thun)
Disciple of C'Thun CAST IF my total crystals<10
Druid of the Flame CAST IF my total crystals=10 my hand has no card named $(C'Thun)
Druid of the Flame CAST IF my total crystals<10
Twilight Elder CAST IF my total crystals=10 my hand has no card named $(C'Thun)
Twilight Elder CAST IF my total crystals<10
Swipe CAST IF my total crystals=10 my hand has no card named $(C'Thun)
Swipe CAST IF my total crystals<10
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