The biggest weakness of a Tiamat Commander deck is its speed.Dragons cost a lot of mana, so early game, you'll more often than not have an open battlefield. Ramp can help, but ultimately, you'll ...

Make Tiamat fall three times -- and inflict enough damage when she does -- and on the third fall she'll fall for good. This will take a very long time, so just avoid her attacks deftly and be patient.