Didn't see this posted over here so...
If anyone is struggling with Nephilim SSS Ranks, found this tip on our sister site.
Credit goes to MarcoDeMars
OK, on each mission, assuming that you already have the SSS for 100% completion, and that u already know the path of the mission meaning you won't lose much time, granting at worst a SS for time, SSS rank will depend mainly on getting a SSS for the style score
If you mess one or more of the fights during the mission, losing your multiplier from a stupid enemy hit, this could prevent you from getting the SSS for style. And reloading from checkpoint is equivalent to a death, and a decrease of 10% for each reload at your final score
But exit to main menu, and using continue (same effect than a checkpoint reload except you don't get back your whole health), doesn't count as a death!
So, if you mess a fight, exit to main menu and use continue. You'll get back at the beginning of the fight, not counting a death. With this tip, SSS ranking is quite a breeze
Edit: You can probably also put the time down to 30/35+ hours.