Crystal Sage can be found on The Road of Sacrifices and is the 4th boss you should face. He is located after the Halfway Fortress bonfire. The sage is weak to lightning and physical damage. You can also poison this boss, but the use of Alluring Skulls (distraction) does not work. Here is a wiki to get more information on the boss:
http://darksouls3.wiki.fextralife.com/Crystal+Sage The only real catch to this boss is that while fighting it, it will eventually "disappear" leaving crystals all over the field. One hit will dispel the illusions that come, but if you destroy the crystals before they spawn it will lower the amount of illusions available. If you are doing this alone, you can summon the NPC who is located right before the door (archway) to the boss. Here are a few videos if you are having trouble with this fight: (credit for videos given before each one)
Video 1: (Credit to PowerPyx)
Video 2: (Credit to IGN)
Video 3: (Credit to Gaming Since Gaming)
Video 4: (Credit to EpicGamerWorld)
Good luck and let me know if there is anything I could do to make this guide better!