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.