Chances are, if you are fighting Shinryu, you'll be doing AT LEAST 9999 damage. No need for Trio. Seeing as Shinryu has a VERY low HP amount, just pound away as needed. The major obstacles to face are his Stone Punch (which will remove your character if it hits), and Shining (which does many hits). Having a high Defense rating is all you should need.

Shinryu also has 2 million HP. Doing 100k per turn WITH Overdrives isn't exactly optimal; or possible.