An easy way to beat those white knight demons is to get up close and either dodge that sword charge or jump behind him with nero and buster the back of him. Simple and very effective!

Below is the entire list of S-Rank requirements for each difficulty level so you can be ranked up there with the best.
There is no reward for getting all S Ranks apart from the pride of seeing your Ranking Table gleaming with S's and the resulting bragging rights that will accompany you on the Network.
An S-Rank is awarded for getting at least two S's out of the Time, Style and Red Orbs. Bear in mind though that the 3rd Rank must be high enough too. You may be fortunate enough to be awarded an S-Rank if only one field is an 'S' and the other two are 'A's though, just as long as they are close to the S-Rank margin.
The Red Orbs are a pain to get S-Rank on as it always requires 100% which usually requires going off path and sacrificing precious time, so it's best to focus on Time and Style with enough Red Orbs to warrant an A-Rank.
So below is a list of all the requirements. I didn't list the Orbs needed as it's always 100% to get S-Rank. Just concentrate on the Time and Style.

Tips:
- Cutscenes and Secret Missions don't add to your timer so don't feel like you have to skip every Cutscene and Secret Mission to get a good time.
- Secret Missions don't add to your Style Points so it's safe to skip them if you're going for the high Style Points.
- In some missions it's sometimes beneficial to fall into a pit or go back a screen to get enemies to respawn (enemies only respawn twice) so you can increase your total style. Just keep your time in mind.
- I recommend trying for S-Rank on your second play through as you'll have more moves and a larger Health Bar which makes it a lot easier. (The Speed Move is a must)
- Using a Continue resets your Style Points back to 0 so beat the whole Mission with one life.
- If you use a Continue then the highest Rank you can get is an A-Rank.
- Using any items will mean you won't be able to get S-Rank.
- Using the Yellow Orb will mean an instant D-Rank no matter what your other Ranks are.

__________________
Mission 1

Human Mode
Clear Time: 0:45
Style: 500
Devil Hunter
Clear Time: 1:00
Style: 500
Son Of Sparda
Clear Time: 1:15
Style: 500
Dante Must Die
Clear Time: 1:25
Style: 500

__________________
Mission 2

Human Mode
Clear Time: 11:00
Style: 6,500
Devil Hunter
Clear Time: 15:00
Style: 6,500
Son Of Sparda
Clear Time: 21:00
Style: 9,250
Dante Must Die
Clear Time: 26:00
Style: 9,250

__________________
Mission 3

Human Mode
Clear Time: 13:30
Style: 10,000
Devil Hunter
Clear Time: 18:00
Style: 10,000
Son Of Sparda
Clear Time: 20:00
Style: 10,000
Dante Must Die
Clear Time: 25:00
Style: 10,000

__________________
Mission 4

Human Mode
Clear Time: 13:30
Style: 6,500
Devil Hunter
Clear Time: 18:00
Style: 6,500
Son Of Sparda
Clear Time: 21:00
Style: 9,250
Dante Must Die
Clear Time: 33:00
Style: 9,250

__________________
Mission 5

Human Mode
Clear Time: 9:00
Style: 6,000
Devil Hunter
Clear Time: 12:00
Style: 6,000
Son Of Sparda
Clear Time: 18:00
Style: 7,750
Dante Must Die
Clear Time: 21:30
Style: 7,750

__________________
Mission 6

Human Mode
Clear Time: 16:30
Style: 12,500
Devil Hunter
Clear Time: 22:00
Style: 12,500
Son Of Sparda
Clear Time: 25:00
Style: 12,500
Dante Must Die
Clear Time: 30:00
Style: 12,500

__________________
Mission 7

Human Mode
Clear Time: 15:00
Style: 7,000
Devil Hunter
Clear Time: 20:00
Style: 7,000
Son Of Sparda
Clear Time: 23:00
Style: 7,000
Dante Must Die
Clear Time: 28:00
Style: 7,000

__________________
Mission 8

Human Mode
Clear Time: 11:00
Style: 6,500
Devil Hunter
Clear Time: 15:00
Style: 6,500
Son Of Sparda
Clear Time: 17:00
Style: 6,500
Dante Must Die
Clear Time: 21:00
Style: 6,500

__________________
Mission 9

Human Mode
Clear Time: 9:30
Style: 10,000
Devil Hunter
Clear Time: 13:00
Style: 10,000
Son Of Sparda
Clear Time: 23:00
Style: 13,750
Dante Must Die
Clear Time: 28:00
Style: 13,750

__________________
Mission 10

Human Mode
Clear Time: 15:00
Style: 10,000
Devil Hunter
Clear Time: 20:00
Style: 10,000
Son Of Sparda
Clear Time: 25:00
Style: 11,000
Dante Must Die
Clear Time: 30:00
Style: 11,000

__________________
Mission 11

Human Mode
Clear Time: 11:00
Style: 11,500
Devil Hunter
Clear Time: 15:00
Style: 11,500
Son Of Sparda
Clear Time: 17:00
Style: 11,500
Dante Must Die
Clear Time: 21:00
Style: 11,500

__________________
Mission 12

Human Mode
Clear Time: 7:30
Style: 7,000
Devil Hunter
Clear Time: 10:00
Style: 7,000
Son Of Sparda
Clear Time: 11:30
Style: 7,000
Dante Must Die
Clear Time: 14:00
Style: 7,000

__________________
Mission 13

Human Mode
Clear Time: 13:30
Style: 7,000
Devil Hunter
Clear Time: 18:00
Style: 7,000
Son Of Sparda
Clear Time: 23:00
Style: 8,250
Dante Must Die
Clear Time: 29:00
Style: 8,250

__________________
Mission 14

Human Mode
Clear Time: 13:30
Style: 6,000
Devil Hunter
Clear Time: 18:00
Style: 6,000
Son Of Sparda
Clear Time: 24:00
Style: 7,400
Dante Must Die
Clear Time: 29:00
Style: 7,500

__________________
Mission 15

Human Mode
Clear Time: 21:00
Style: 11,500
Devil Hunter
Clear Time: 28:00
Style: 11,500
Son Of Sparda
Clear Time: 34:00
Style: 12,250
Dante Must Die
Clear Time: 41:00
Style: 12,250

__________________
Mission 16

Human Mode
Clear Time: 18:30
Style: 10,000
Devil Hunter
Clear Time: 25:00
Style: 10,000
Son Of Sparda
Clear Time: 30:00
Style: 10,650
Dante Must Die
Clear Time: 37:00
Style: 10,650

__________________
Mission 17

Human Mode
Clear Time: 13:30
Style: 17,000
Devil Hunter
Clear Time: 18:00
Style: 17,000
Son Of Sparda
Clear Time: 21:00
Style: 17,000
Dante Must Die
Clear Time: 25:00
Style: 17,000

__________________
Mission 18

Human Mode
Clear Time: 7:30
Style: 5,000
Devil Hunter
Clear Time: 10:00
Style: 5,000
Son Of Sparda
Clear Time: 11:30
Style: 5,000
Dante Must Die
Clear Time: 14:00
Style: 5,000

__________________
Mission 19

Human Mode
Clear Time: 22:30
Style: 12,500
Devil Hunter
Clear Time: 30:00
Style: 12,500
Son Of Sparda
Clear Time: 50:00
Style: 20,000
Dante Must Die
Clear Time: 60:00
Style: 20,000

__________________
Mission 20

Human Mode
Clear Time: 2:30
Style: 5,500
Devil Hunter
Clear Time: 3:15
Style: 5,500
Son Of Sparda
Clear Time: 3:45
Style: 5,500
Dante Must Die
Clear Time: 4:30
Style: 5,500

For Nero, don't forget to use your charge shot. Level 3 can be very handy for really boosting your style ranking. A Level 3 charge shot followed by a Buster is usually enough to bump your style up a whole rank and sometimes more.

With Dante, if you're really struggling to beat the combat ajudicators, use Lucifer and repeatedly use the Pin Up move. Unlike other attacks, you can use this move over and over and it will still raise your style ranking.

You can unlock Super Nero and Super Dante costumes by beating Dante Must Die mode.

You can unlock various art-work from the game by doing the following:

The Two Heroes; 29 character images - Beat Human or Devil Hunter mode
The Cast - Beat Devil Hunter mode
The Demons; 12 publicity pictures - Beat Son of Sparda mode
Demon Invasion - Beat Dante Must Die mode
The Ladies of Devil May Cry - Beat Heaven or Hell mode
Light from the Demon Blade - Beat Hell or Hell mode

To unlock the different modes (difficulty levels), you will need to do the following:

Son of Sparda mode - Beat Devil Hunter mode
Dante Must Die mode - Beat Son of Sparda mode
Heaven or Hell mode - Beat Son of Sparda mode
Hell and Hell mode - Beat Dante Must Die mode

Unlock the Bloody Palace by Beating Devil Hunter Mode

Even whilst the credits roll, the game doesn't end. Whilst the credits are rolling you will need to protect Kyrie from being hit. If she is hit, the credits will end prematurely, however if you succeed in protecting her for the full 90 seconds, you will get to see the rest of the ending.