You will also see him kneel on the ground and fire at you, giving you the perfect opportunity to dodge in behind him and attack. Use your distracting spells to divert his attention and then charge in with your attacks. You need to get behind him and attack from there to deplete his HP quickly. Moreover, he will summon his minions into the fray. He will also put his shield aside from time to time, giving you some room to attack.

Similar to the First Warden, Commander is also hidden behind a massive shield.