There are 4 types of mercenaries in the game, each type fullfills a certain type of role in the game and excels at doing these roles.
The main roles are:
Tower Destroyers/ Tanks
These type of mercenaries have as favorite target: Defensive Buildings. They have high hp, low attack damage and moderate movement speed, they will always attack the defensive towers until all are destroyed, when all the towers are destroyed they will attack the next closest building.
Only Pharaoh Warrior are different from the group with their low hp and high attack damage, their role is to support the main tanks in higher levels when the towers are fully Fortified.