Diamond Dog starts with a Shield α, which reduces physical attack effectiveness by 50%. Use the Neutralizer to fix that.

During the battle, Diamond Dog can cast Shield β, which reduces damage from your attacks AND throws some of the damage back at you. Have Poo cast Shield Ω to counteract this.

Use the Night Pendant, Sea Pendant, and Star Pendant if you have it, to protect against Diamond Dog's PSI Flash Ω (called "glorious light").