Sigh… Sanctus is one of the best examples of what you’d call baaad boss design. If anyone ever finds him fun, you're a weirdo. Also, before you start it, make sure you have Snatch LV3!! (LV2 at least)

1. Sanctus remains invincible until you break his barriers. The fastest way is to snatch yourself over and then spam Aerial Combo.
    
   
2. The moment it breaks, Sanctus will retreat backwards really fast, stopping only to attack. You can pretty much avoid all his attacks by jumping but the problem is getting to him fast enough before he recovers his shield.
    
3. You can do this by snatching all the way towards him (works on the easier modes) and then whack away at his 'second barrier' until he's stunned. On the harder modes, you might have to learn to snatch only when he's open (while he's executing a move).
    
4. Good thing is, this second barrier doesn't grant him invincibility; he still can get damaged by your attacks, like for example Charged Shot. It also takes less hit to break so just keep trying.
    
   
5. Once his second barrier is gone, Sanctus will collapse and be entirely vulnerable for a while. Attack while you can, ending with a DT’ed Buster if possible and mashing [] (shoot button) for additional damage. You basically repeat these 3 steps until he’s got only 3 bars of life left (5 bars on the harder modes).
    
   
6. Once down to 3 bars of life, Sanctus will call upon the help of the Savior. It’s pretty lame though, cause’ he only tries to crush you with his fist once, which you can easily side roll away from, before Sanctus comes back for you to repeat the above 5 steps again.
    
   
7. The only difference is, Sanctus gets even more defensive this time. Oh lord. After breaking his barrier, he will retreat backwards like mad, and will start attacking with fireballs (the fireballs can actually be countered by shooting at them) and such that makes it harder to snatch yourself towards him without getting hit and thrown way back.
    
   But since you still need to stun him before he retreats, you really have no choice but to either run towards him or snatch your way back. Be warned that this is going to be very frustrating. If you fail to stun him in time, Sanctus will float away and the Savior will do the fist attack again once before Sanctus comes back for you to repeat all the above steps.
    
8. Luckily, thanks to Marvin, you can Buster (press O) the Savior's fist before it hits you and Sanctus will get knocked out of the Savior and rendered vulnerable instantly! This is your ultimate life-savior (oh the pun!) for DMD mode.
    
   
9. Otherwise, hopefully you can get in enough hits to stun him when he pauses to cast a spell, and when it breaks… KILL HIM. Release DT and everything! If he has low enough health (less than 2 bars) quickly just press O for Buster in case Sanctus recovers and you have to do it a second time. Urgh. Good luck.
   

Video demo on DMD | Mirror link on Youtube [11 mb]