Simply unlock all achievements for unlocking this one
If you Go for the achievement where you have to unlock all skills and you are wondering That there is one skill still locked you have to travel to hirata estate
Jump off the cliff and head into the water, take a right where you can find a dude
In a box ( near some rocks)
He will sell you the Last skill what cost some treasure carp scales
It will cost 9 skill points so be aware That you hit this guy on time otherwise this Place is locked After you progressed to far in the Story
Greetings BadCranky