If you find that you haven't obtained a S rank, dont worry.
Sync is really just being alert at every cutscene, and being able to get excellents at near every occasion. So you should S this one easily
Time and points can be tricky, if you complete a chapter and at the results screen, hit Y and you will see how points are got and which ones give the most. The problem is if you rush too quickly, your battle points will be too low. And if you spend ages upping your points, your time will be too low.
My advice, obliterator gauge as you know will help out majorly. Since most of these are boss fights, what you are aiming for is to unleash as many heavy attacks as possible on the enemy, and to use special attacks alot (when the enemy is down and a Y prompt appears) Also once the burst is full, try some more heavy/ special attacks. And make sure you try to get all the boss counterattack sequences done too.
If you aim for an S in time and sync, and an A in points, you will get your S's in no time.