Well I do this every time when I'm in battle with them making them partially immortal to any damage.

What you do is:
1. Use Guardian
2. Hurry up when he is using guardian and Cast a spell like Thunder Blade OR Healing Stream
(THIS IS THE GLITCH WHEN THE ENEMY HIT YOU WHILE CASTING THERE WILL BE NO DAMAGE WHAT SO EVER)

[Hint: Won't work for the spell "Judgement"]

3. Hold out on the spell and it will say "Charge" on top of the face of Kratos Or Zelos But Only If you have the EX SKILL: Spell Charge.

4. Look what happen now your just standing in place and still no damage from the enemy. Now you can use any combo on them and after you cast out your spell etc. The Glitch will be gone and your back to yourself getting hit again and taking damage. Then Do it Again and choose which ever spell you want.

5. Though you can use as much as you want in battle. This really help me out when I battle Abyssion all by my self with only Kratos AS ME and Sheena Only Alive. Sheena couldn't really revive a lot and right in the middle of battle I figure out how to use it, I manage to beat him at Mania Mode and revive Genis and Raine. I finished him off with a Prism Star. And It was the first time I battle him in Mania Mode.

{Kratos:lvl 78, Sheena:lvl 75, Genis:lvl 79, Raine:lvl 78}

Hope I was Helpful! Reply If You Want.