Tiamat is the penultimate boss of Makaimura for Wonderswan, a demoness which first attacks Arthur in a humanoid first form, and then assumes a very different second form, resembling a three headed dragon.
Tiamat's first form attacks by slowly moving forward slowly and then swinging her sword at you. Sometimes her slash will create a wave which will run through the ground as well which you will need to avoid. She can only be hurt when the sword is being swung out.
Second Form
Tiamat's second form attacks by spitting fireball through her three headss. To damage her, get on the steps at the left of the boss area and aim your attacks at the heads.