Simply play normally, you'll eventually get the set amount, just learn the locations of the precious stones and do the medals to get some extra BP.
Or if you're really impatience or if you want to buy all the skills early in the game you can do the following grinding method:
In chapter 2 at the beginning of Calire episode go to
*** Spoiler - click to reveal *** the chopper with Gabe, that will create a checkpoint, now go back to the starting place (the bar) and break all the wooden barrels and shine all the treasures inside the bar but don't grab anything, there should be 2 barrels 1 in the second floor and 1 in the back room, the shining treasurers should be 1 at the left side of the jukebox, 2 behind the bar and 1 on the left side of the tool table, then go to the shed outside the right part of the bar (where there's some steel barrels) there's another shining treasure, once you think you have found good precious stones (preferably blues or reds) go outside to where Pedro should be, he should open a stoned door with the drill, go inside and in the back room there's another shiny treasure, now go outside and in front there's yet another shiny treasure, now go directly to the stoned door that Pedro needs to drill for you to get the fuel canister, now finally head back to Pedro again in the house where he tore down the first wall and that should create a new save point, now if you light all shiny treasures and broke all barrels and boxes and found a decent amount of precious stones without picking them, once you do that second checkpoint you can go ahead and pick them all once you do just simple retry as many times you want.
That work's for getting a lot of Bp and getting everything in the game and the achievement.
Also do this on survival as it gives better precious stones.