This achievement is for beating the very first boss of the game. The first phase of his health (he will change at around 1/2 - 3/4 hp) is generally simple. Dodge his strikes and
BE CAREFUL of his swipes. This boss is weak to frost and fire, resistant to dark and bleed while bein immune to poison and toxic. Here is a link to a wiki for more information on the boss:
http://darksouls3.wiki.fextralife.com/Iudex+Gundyr Below are some videos that will help:
Video 1 (deprived class):Video 2:Video 3 (thief class):Video 4 (Knight class):
Hope one or all of these videos and tips helped! If you have any tips let me know.