Are you following the directions at the top of the grid when you are selecting moves? After you defeat him once, the only way to win the fight is to destroy the three statues or totems in the corners. Two are in the top and bottom left corners and the third one in in the bottom right of the grid. Once those statues are hit, the fight is over and you will have won.
What might work is: Before you take Chumba down the first time, postion your companions so that they are within reach of each statue. Now when you take Chumba down, the statues can be hit the very next turn with no delays. Hope this tip helps.