The game has 12 chapters.
For this achievement you only need to finish half of the story which is somewhere in chapter 7, where you play as Takeda.
(I know 7 is not half of 12 though, but chapter 7 is where it unlocks and if you go to chapter select you will see, that chapter 7 is in the middle of the campains timeline).
If you should still struggle with some of the fights, there is always the option, to change the campaigns difficulty in the game options under:
"Options" - "Gameplay" - "Campaing Difficulty"
to a difficulty that suits you more !