Select Claire story line of Episode 2. No difficulty and mode restrictions. But Invisible mode is highly recommended.
After you skip the intro CG, you have to find gasoline and battery for the chopper. Wait the dialogue ends until the upper right corner of your screen shows your next mission objective, which means the progress has been saved.
Switch to Moira to open the military crate behind the chopper. You will randomly find a Sapphire or Emerald, which gives you 500BP or 1000BP.
There is a house behind a broken wall attached to the front door, and you can find another military crate there. Similarly you will also find a Sapphire or a Emerald in it。
Press ESC to open game menu, choose Retry, the game will warn that you have unsaved progress, but your obtained BP will still count.
You can obtain 1000-2000 BP in about 1 minute after you have been familiar with this method. Don't open military crate before the mission objective shows up, or you will still get opened crate after you retry. The reason I recommend Invisible Mode is that in Story Mode the custom parts will be in the crate instead of gems.