When serving in the Army I used to break-in my "Boots Combat High" and before that "DMS" by removing the plastic insoles, placing the boots in a bucket of boiling water with a cupful of washing up liquid and weighed down with a house brick.
I'd leave them to soak in the water overnight. I'd then take hold of one boot by the shaft and use it to pulverise the other boot, on the toe, along the lace holes, up the stem etc. I'd then soak them again with boiling water for another night. Then I'd drain the boots for about ten minutes, and without replacing the plastic insoles, put on two pairs of thick socks and then put the boots on, blacken them up with Kiwi Boot polish and wear them until they were dry. That usually did the trick! Occasionally there would be a sharp, hard piece of leather on the inside of the boot where the stitching, tongue, boot met, if that was still hard I'd use a metal hammer on the outside of the boot with a buffer on the inside. That usually broke them in!