Use the following trick to defeat Sanctus inside of Sparda in Mission 20. While Sanctus is airborne in his shield, get close and break it with sword attacks just like earlier in the game. However, he will sometimes block with the Sword of Sparda. When he does this, use the uppercut technique to break his guard. If you are skilled you should use the Devil Trigger while he is in the air to use a special combo to force him to the ground. If you fail to do the air combo just let him fall to the ground. You may be able to get a few sword-based shots in but do not try to rely on this because he may escape. At this point, use the uppercut in Devil Trigger form, if at all possible. Afterwards he will go back to being airborne again, in which case you should drop the Devil Trigger. After you have broken his health down to about the 10% to 15% mark, he will use a charging sword attack (however, if you have perfected the aforementioned combo, you may bypass this). Defeating him requires timing. At the precisely the correct moment of his charge towards you, uppercut and break his guard. Again, as soon as it is broken get into the Devil Trigger uppercut and finish him off. In the second battle against Sanctus, as the False Prophet, timing is once again crucial. All you must do is uppercut his attacks away.
To easily drop Sanctus' Shield during either of his encounters, make sure you have a full Exceed gauge (Ex-act or Max-act helps). Approach Sanctus, jump, and perform an Ex Calibur. If the full combo hits, his shield will be down. You can now proceed to make him fully vulnerable to attack using the above method, or however desired.