Use the following trick to kill Tenrai's second form in less than fifteen seconds. This requires one Binding spell, two Fire spells, and the Wrath Of Heaven ability. As soon as you regain control of Ayame, cast the Binding spell. Stay in the circle, and wait until Tenrai gets "bound" when he runs up to the edge. Cast a Fire spell to knock him back. Repeat this step with the remaining Fire spell. When he returns and gets bound again, use the Wrath Of Heaven to defeat him.

The following is a much easier way to kill Tenrai's second and third form on the last level. As soon as the intermission sequence ends and Rikimaru can focus through his right eye, perform Wrath Of Heaven just before Tenrai runs towards you. Do the same thing again for the third form. If done correctly, there is no need to bother with the Bind or Fire spell.