A unit must be led by an eligible leader. On any given mission, there is a limit to how many units you may deploy. When one of your units meets with an enemy unit, combat ensues, continuing until each character uses all his attacks and the side that causes the most damage is declared the winner. The loser is repelled, and the mission continues with the possibility of these two units meeting each other again. In order to cause the most damage, units must be constructed carefully.
Wizards go in the back row, so they can concentrate and get off more spells.
On the other hand, it would probably be best to put the large golem up front to crush the opposition. Each unit must be arranged in a three by three grid, and no one can be placed in a square bordering a large character. Hence, you will never have a large character in the dead center. Position of a character within a unit not only determines the number of attacks and amount of damage dealt, but also the type of attack performed by some characters. Place a Paladin in the front and he will slice into the opposition, but if he is in the back row he will heal his wounded comrades.