Heyyyy there,
Gust is a great card, but I would choose Never Lonely instead. It gains beneficial damage once Dementia settles in (bonus dmg for each shadow guem). Gust is only valuable if you're able to pump your SP which means you need Turned to the Shadow Side ASAP.
Shadow of the Night x3 would go in my deck, the 3 turn def +2 bonus is greatly welcomed, not to mention able to recycle a shadow spell each time.
Cloning to draw a Shadow Agony from the discard pile is hard enough, if to be meddled by Shadow of the Night would only make the possibility worse. But I recommend Cloning without Shadow Agony involved.
Ziloween isn't worth the card slot you're using. You already have dementias to get the bench.
I'd go with Shadow Tentacles x3 cuz 3 naked characters makes the whole game empty, except for your mages.
Cut down on the Double Clay, there's no point going full defense when you're barely dealing damage. Consider that the opponents will take their time to buff up as well.
Prestige, hmmm, I always hate having this card in my deck of Mages. It's because mages are usually suited to playing spells that help them defensively, or offensively, due to their initially weak attack and poor defense. Playing a Prestige at one point wastes this phase, for a chance to maybe remove 3, 2 or just 1 card from the game
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