This one is very straightforward, just go into the dungeons and kill mini-boss and boss enemies to earn Judge Blood(the golden blood droplets). Next time you die after obtaining one you can go to the pedestal on the left side of the hub area(The Chiming Tree) to check out the skill tree. For the completion we'll be focusing on the first branch, the one in the bottom left corner. This contains largely defensive skill options but they are also the cheapest skills to purchase.
You'll want to buy Oak Skin, Ground Pound, Resilient and Dirty Barry.
After these, to speed up later branch achievements I recommend getting the following skills ASAP; Combo and Crystallization. The first is found on branch 2(bottom right branch). It is the second skill, available after taking Lightning Travel. As your combo increases during fights the amount of Blood you obtain per fight is boosted. Fighting well pays well! This pairs very nicely with the second skill of branch 3, Crystallization. Every 1500 Blood you have earned gets turned into a Judge Blood when you die and respawn at The Chiming Tree. This means even if you fail to defeat a boss every run, as long as you keep your combo high and earn enough blood you'll always be progressing a little at a time.