After some digging, I found a few Gameshark codes for Yu-Gi-Oh! Forbidden Memories that have been verified to work. Here's a feature related to these codes:
(SLUS-01411), these GameShark codes are widely verified and cover essential gameplay enhancements. ⚡ Essential Battle Codes Use these to gain an immediate advantage during duels. : 800EA004 270F (Sets LP to 9999) Instant Win (Opponent 0 LP) : 801A7D16 0000 yugioh forbidden memories gameshark codes verified