Certain formations are useful against gunfire, while others, such as the square, are only good for staving off cavalry. Of course, while the unit is tightly packed together with bayonets out, the gunfire will slaughter them. They are the real muscle of Waterloo , and all other units are support. Artillery, for example, is clearly a support unit. Cannons have incredible range, but of course their accuracy is highly questionable.
Artillery is most effective at breaking unit cohesion and morale.
Cavalry is death on horseback. If infantry or artillery are unprepared for a cavalry charge, they will be cut down like wheat. However, cavalry are weak units and must be used with care. The last unit is a noncombatant, but is absolutely essential to move many units at once, as well as keep morale from breaking. The leader unit is a specialty one.