To constantly have your shield up simply press the B button rapidly. This will keep you from taking damage from enemies, But you will still take damage from environmental hazards. (e.g. Lava, Swamps, etc.) NOTE, Do not hold down the B button because you will just start charging your Elemental energy and be vulnerable to attack.
This Cheat Only Works With Blue And Black. Hold B Press Y Right After You Charge Your Energy All The Way Up.