Instructions
Right-click the Creeps to have your champion auto attack them. Auto attacking will use your base damage statistic to inflict damage. When the enemy creep runs out of hit points, it will die.
Use any area of effect abilities on the entire Creep wave. This will deal damage to the entire wave instead of killing them one at a time.
Focus your attacks on creeps with low health, this will decrease the numbers of the wave making the rest of them easier to kill. You only obtain money and experience for killing monsters, not for damaging them.