Place the correct allies in the corresponding locations. For example, your starting allies includes Little Monkey as a ranged attacker and Big Monkey as a melee attacker. Big Monkey does not have very high attack power, but has high health. Enemies usually will not attack ranged allies. Place ranged allies first, followed by a melee ally. Enemies will target the melee ally, allowing your ranged allies to finish them off if they are close enough.