Here is the full list of all 12 Shiner achievements.
Beat Gatekeeper.
Beat Wally.
Beat Bouncer.
Beat Gresh.
Learn every attack.
See ending 1.
See ending 2.
Lose a fight with Skrimpo.
Beat Sharpnut, Hardened Thief while knowing 6 or fewer total attacks.
Beat Leodora before they run out of Counter uses.
Beat Ranch before they run out of Divine Body uses.
Beat Wally without using Fireball.