EDIT: It appears, according to Ratpoizen and Raezie that this is not possible on Mission 11, and as such has to be done on Mission 20.
If you would like me to add techniques on how to dodge each individual attack, let me know and I will add them to this guide.
The only pre-requisite is that you are used to charging a shot with Nero whilst being able to fight. This is invaluable to gaining and also maintaining style meter throughout the fight.
For this achievement you will need to beat Sanctus with a combo of SSS on Son of Sparda difficulty or higher.
I found this easiest to do on mission 20, mainly as he has less room to run away from you. I'm also not sure if the boss on mission 11 would also give you the achievement.
Anyway, I just wanted to share the strategy I used, as I'm not a fantastic player, and a lot of the no damage/stylish runs you see on youtube etc can blind you with science instead of allowing you to interpret a more simplistic approach.
To start, you *can* take damage, however if you take damage and Sanctus is on 60% hp or less, I would recommend restarting the mission as it would be nigh on impossible to rescue you style meter in time.
So to start, I'll explain my combo pattern, complete with a video clip from my Xbox One (which isn't perfect, I do miss a couple of hits, but it still shows that even with that, I'm still getting up to SS and SSS).
Let's assume, you've already taken down his shield and ended your combo with a throw to punch him into the wall, as, if you're going for this achievement, you'll know his attacks well enough, or you will learn them well enough whilst getting the pattern down for the style meter.
From him disappearing, after hitting the wall, run over to him and jump. Slash him 5 times with the basic
air attack, then hit the floor to dodge his satellite. From here, chase him, jump and hit him once more to break his shield.
After this, find your opening and jump to hit with with another
in mid air. One of three things will happen.
1) He will guard with the Sword of Sparda. If this happens, you have ample time to jump back up at him, hit
to break his guard, and then jump at him again and continue with step two.
2) You will hit him, if this happens, hit
a second time then knock him to the floor with
aka the move "Split" with the Red Queen. Then hit him with a Streak which is the same button combination whilst on the floor. Grapple him back to you with
, hit him with a level 3 charge shot and grapple him back again. From here hit the
combo, grapple him back then grab him with
to punch him in to the wall.
3) He may come to the ground and do a circular slash with the Sword of Sparda, if this happens, as soon as the sword disappears, let off a level 3 charge shot, then hit him with the streak. You will then be able to go into a combo before grabbing him to end the loop (This is also depicted in the video at the end of the guide).
The next part isn't necessary but if you can get the timing down it will help your style meter build so much more. As you punch him into the wall, as soon as he starts to drop, you get back control of Nero, let go a level 3 charge shot. This, and the explosion seconds later both increase the style meter and help you get up to the S, SS and SSS ranks. You won't land it every time, but it's always worth attempting.
From here it's a case of rinsing and repeating the same process, and towards the end you can start using devil trigger during the combo to help that style meter go up a bit higher.
Here is a video showing an example of the loop, and even though a couple of my attacks whiff, the meter gain is still substantial due to the charge shots landing.
Once you've done this a few times, either, you will kill him before he goes into his final stage and may get the SSS rank, or he will have a slither of health left, and you'll have to adapt to his ending animation.
If the latter is the case, the only way to finish him with an SSS is to land a level 3 charge gun shot, whilst he is winding up his lunge, and then catch him with your buster (
) as he comes to you. I found the way to time the buster, was that he will edge back slightly, before lunging, as he does, you'll be ready and as soon as he starts moving forwards, tap
. Again, you won't get it everytime, because like me, you're just human, but that method will allow you to get it more often than not.
Again, here is a video to this effect. After this point, the achievement then pops.
I hope this guide was helpful. Any feedback or ways to improve would be much appreciated, as would a thumbs up.