The following are known Action Replay Codes for Monster Rancher Battle Card GB on Nintendo Game Boy / Color (GB / GBC). These codes were also known as GameShark Codes and in most cases will work on both devices.

Device Compatibility: Action Replay & GameShark

Infinite HP
015ff8c1
015ff7c1
015ff6c1

Infinite GUTS
015f15c2

Beat 1 Character to Win
0100fbc1
0100fac1