Looking Glass Knight NG+7 Covenant of Champions, No Damage

Dark damage is very powerful against this boss, he has aa very high lightning resist so using Resonant weapon instead of sunlight blade is the best choice. Another good strategy is to cast toxic mist on him twice when he is summoning a mirror squire, this can take out about a third to a fourth of his health.
The spear and shield squires are much more troublesome than the greatsword wielding mirror squires. The spear squires will rush you and are very aggressive while the greatsword ones tend to be slow and get stuck on the boss. Extremely fun fight, one of my favorites in the base game.
A good way to set up the red tearstone ring before the fight is to run back through the Twin Dragonriders room and go to the stairs where there is a lone Syan Knight. Block two attacks from him with the vessel shield then run away and you should be set up for the ring. Also he drops 3k souls which is more than enough to use resonant weapon.

Dark Souls II: Scholar of the First Sin