Rainbow or Layer or Darwin method is a commonly used attack scheme and use only requires that you send meatshield units, namely Swordsmen, Warriors and pikemen to cover your Main attack Force, namely Archers.

The way this system works is that each group of units will only attack one other units and in most cases either the Fastest or Most Valuable unit type. Sending meatshields in rainbow attacks stops the enemy from attacking your main force and attacking those weak units before attacking the better damaging unit.