Use the WASD keys to move the Wizard, and use the F key to fire your spell. If your run out of health, then game over. If the ghoul runs out of health, then you win. If you run out of energy, then you will be immobilized until you have more that 1 energy. By the way, you use 10 energy every time you fire a spell.
The ghoul only takes damage every 2 seconds if it is touching your spell. So don't spam!