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