Defeating the Dodai can be a long process and requires patience, especially due to their extremely high HP. However, the reward is greater than simply defeating the three opposing fighters. They are best attacked in Round 2 against Cawer, where there's a chance he'll unintentionally heal yours with Rejuvenate. Naturally, as they possess the Fire element, Ice and frost attacks work the best against them. If using Ryu, combine the Flame and Reverse Genes to create a Frost Whelp (since you don't have access to the Frost Gene itself yet), and attack with Frost Claw until you run out of AP. That should get you to about half of their HP, if not more, depending on how much AP you have. From there, use Skills like Jump, Snap, and physical strikes to whittle down the rest of its HP. If not using Ryu, use Momo's Bazooka Frost Chrysm or Nina's Ice Blast to do the job.