Because of the skill level for this metcon, what we want to focus on from the get-go is breakup strategy. Squat snatches are full range of motion, and chest to bar pull-ups (as well as regular pull-ups) add onto that highly taxing range of motion. Therefore, if you finish the round of 21 and you're exhausted, 15 will be your rude awakening. So expect it to burn, just don't burn out too quickly and have in mind your break up strategy. No matter how much it bites, just keep moving. (See what I did there? I'm a friggin poet.)