To unlock this achievement you need to defeat the first boss of the game Zhang Liang. Imho is one of the toughest in the game, so don't worry, it gets easier after.
I made this video to help you understand better how to defeat it:
Aside from the video, some general info that are worth mentioning: you have two main stats, your attributes (wood, metal, water, gold and fire) and your morale.
Your morale is set to the area you in (in this case it's 10 if you start straight from the resting flag and you have unlocked all of them). Morale determines how much damage you suffer and how much damage you do: higher morale means higher damage. So if you find yourself in a predicament, you can farm a bit some of the mobs outside his area to start the fight with 11 or even 12 morale. You can make the boss drop some of his morale when you deflect his red attacks (in the video it'll drop from 10 to 9). So that means deflecting red attacks is very important!
Be aware of your ki bar (the one you have in the middle), if you are in the left side of it, you do LESS damage and you can't use your spells in some cases, as well as you may get stunned if it's too far on the side. On the contrary, if it's on the right side, you deal more damage overall, crits included. So try not to spam too many spells, unless you are very sure of your dodge/deflect game.
Wo shares some similarities with Sekiro, which means it's better to be on the offensive (deflections and counters) rather than dodging a lot (like Elden Ring/Dark Souls).
Hope this video and those general tips helped you!