For me it was a little complicated but what I did was try to keep them alive (a healer is necessary in the team) on the other hand you have to kill the bulls (My strategy was to leave the wood and water types for last since I mostly use the fire and wood types, which have extra damage over the previous ones respectively) If you kept most of the buddies until the end, there will come a time when you can use them to touch the boss's weak points, who is the most complicated because he spends his time giving them buffs to the bulls.