After the first set of levels are completed, you will receive new heroes as rewards. This gives you a better chance against the enemies that appear very fast. If you do not have any melee allies in your party consider using your heroes. Try to keep moving to be useful. Repeatedly send them to where enemies are located. They will automatically attack when in range. Some heroes are more suited for a tank role than others. For example, Monkey King is better suited as an attacker, while Pigsy makes a better tank.