This is an unmissable story related achievement. You will fight the Looking Glass Knight at the end of Drangleic Castle, which is accessed once the four primal bonfires have been lit or once you have 1 million souls (first playthrough).
The Looking Glass Knight can be difficult as he summons either an npc invader or a player invader during the fight. If he does this then you should kill them first, otherwise you wont have many openings to attack.
He is mid-speed with easy tells for his attacks. Obviously your tactics depend on your build, but I usually dance around him and poke him occasionally.
If you are a mage then you may struggle because his shield reflects your magic. However, stand close to him, do a roll to go behind him when he attacks, and then use your spells.
Thanks to mage strats from Hikaru No Go.