The looking Glass Knight is located in Drangleic Castle past the Central Drangleic Castle bonfire, accessible after beating the pair of Dragonriders. The boss itself telegraphs his moves and is fairly easy to avoid, however if he connects, he hits hard. Avoid attacking his shield arm, as the shield is impenetrable and most melee attacks will simply bounce off. The shield also has a chance to parry and reflect spells, so spellcasters beware. He also has several ranged lightning based attacks, so it may be advisable to take a lightning quartz ring into the fight. The Looking Glass Knight will summon an NPC to assist him in the fight. There is a chance that another player will be summoned instead of the normal npc, making this fight potentially much harder. This is done by a player laying a red sign soapstone down in the hall leading to the boss, so should you see any red signs on the ground, and do not wish to engage in pvp in addition to the boss, you may want to tackle this fight offline. Should another player be summoned, direct all of your attacks and efforts and them and eliminate them as soon as possible.
Additionally, you can summon an NPC, Benhart of Jugo, to assist you in the fight. While Benhart does not actually contribute to the fight that much, he provides a wonderful distraction, allowing you to dish out damage and heal if needed. If you are attempting to get the achievement for inheriting Benhart's equipment, then this is one of the fights that counts towards doing so (you need him to survive 3 out of 4 possible fights, this being one).