Try ballad of revolt ("kjempeviseslåtten") by Harald Sæverud. The piece is starting quite, constantly building up to a huge climax in the end. There are a lot of crazy octaves in both right and left hand. My octaves improved a lot after playing this, it is also a really nice piece with a lot of aggression. I would say the difficulty level is grade 7/8 depending on how fast you play it.The piece was originally written for the norwegian resistance army who fought the germans during ww2.