The method that I've used successfully on a small batch of seeds is to put them into a bag to rot for a few days. Once they get mushy you spray them down with a hose and all of the flesh washes right off the seed husk. If you try to wash the flesh of a non rotten mango, nothing happens. Once you wash the flesh off, let the husks dry in the sun for a few hours. They will then be much easier to open.
For a larger scale, I've seen a nursery put thousands of the turpentine into 25g+ containers and let them rot and then put the fruit into a small cement mixer to wash off the flesh. Then after the flesh is off, dry them in the sun and then open them.
If you've done all of that already and you are asking specifically about the tools used to open to endocarp, I just use my hand pruners.