For Gemini, Quintessence for your high damage creatures is a must. Otherwise, he will just use Twin Universe on them and kill you faster... also healing is vital (i.e. Miracle, Shard of Gratitude, Feral Bond, Vampire, ect.). I suggest taking Quintessence (the card), Ruby Dragons and momentum (to bypass his Phase Shields) so that you can deal maximum damage, as well as Protect Artifact, Electrocutor, either Fire Buckler or Titanium Shield, and a few of the aforementioned healing cards to beat him.
Also, finkel, i would not recommend taking mind flayer because of his Twin Universes. Unless you want to Quintessence them.