The first step in defeating this robot thing is not to attack but charge everything to max first. Charge first your defense, then your magic, and finally your attack with your first 3 turns.
Afterwards just use your regular attack on the true guardian. Do not use your magic attacks, you will need them for healing. When true guardian gets you down to a low HP, use your heal to gain back some HP. I suggest never letting your HP falling below 500. As true guardian attacks you, your overdrive will charge up. After it charges up to full, use either the sword dance or power blast(it doesn’t matter which one) to make a large bit out of his HP. Just rinse and repeat this strategy until true guardian is toast.