The Trial Deep is the last stage in Neverending Cat Story.
Max out your wallet with Wall Cat. Then spam everything. When the Boss comes out, fire the Cat Cannon, that should remove the peons (Croco and Snache) so that Valkyrie and Dragon can hit the boss.
Beware when using Bahamut Cat. Due to the range, he can be knocked back while charging up his attacks. However, If Bahamut is Level 20 or higher, he will be able to land a hit on Master A. (even on 3-star) provided he is not distracted by the other enemy units.