Hello there I am rather new to MGR but I found a really easy way to cheese the game and get tons of BP.

When you play MGS you can push options/select to open up the codec. The codec will give you some options as who to talk to and a customize mode which will allow you to spend you BP mid mission. When you finish up there it will 'reset' you to the last save point you have but it will also respawn all enemies and chests ( except for endurance +1). Now the great part is when you use customize instead of reset on the menu you don't loose items or BP you already have.

There are a few places in the game that have 5000 BP 'blade chests' ( which you have to use blade time to open) such as mission 2 by the ferris wheel.

So you go and get the 5000 BP chest, go into codec, choose customize mode, exit out asap. Then rise lather repeat. I also use this if I missed a left hand ID to redo it. If you just use reset you loose all the BP you have gained until that point.

Hope this helps a little this is such a fun game, and I LOVE running around with the grey fox skin and the fox blade..... so much fun.