Rather than showing a video or explaining how to get S rank specifically for this stage or achieving 100% specifically on this region, I feel it would be more valuable for you to understand how S rank is achieved in the game to have an easier time overall with the rest.
However, if you prefer videos, this YouTube playlist shows where all the Red Rings are while also getting S Rank (includes bosses). I am not the creator of these videos, but I leaned on them heavily for methods to beat certain stages getting decent amounts of points without running into "Time Up!".
https://www.youtube.com/playlist?list=PLzW8CnEl03XoVpGJAzs6A...Getting S Rank on any stage, including boss stages, is based entirely on your Score, and the amount of points you need for S depends on the stage. They generally hover around the 1,200,000 mark but some stages need more than 2,000,000. You can see how many points you need for the next rank during the Results at the end of the stage, on the left side of the screen.
While playing stages:You get points for performing the following actions which go straight to your score;
- Collecting rings (100 per ring)
- Destroying enemies (at least 1,000 per enemy, larger enemies award more, respawning enemies only award points the first time they're destoryed)
- Collecting the blue rings from the Cube wisp (10,000 per blue ring)
- Using the laser wisp on the diamonds (Amount TBC)
You also get bonus points, which get queued on the right side of the screen by performing the following actions;
- Destroying crates as the Drill/Cube/Spike/Berserker wisp (3,000 per crate),
- Collecting Red Rings (30,000 points, even if you've collected them before)
- Side Stepping during specific sections of the stage, usually chase scenes or where you have to knock enemies off wheels (1,000 per Quick Step, up to 100,000 total for the stage)
Note: As of the time of writing, there's a bug where these bonus points are being mislabelled, often as "Extra Ring Bonus" or "Special Bonus", but the score applies.
At the results screen - The time it took you awards points the less time you take. I believe it awards points based on the difference between your take taken and the time for the "Time Up!" message, which varies per stage.
- The amount of rings you are holding at the results screen.
Unlike most other 3D Sonics, death does not necessarily throw you all the way back to the last checkpoint, nor does it reset your score or rings. But it will revert your score, boost meter and rings to the values they were when you ran into the checkpoint, and revert any pickups you collected after the checkpoint, while not reverting your time. This means you can possibly still get an S rank after perhaps a couple of deaths, but generally you'll want to avoid dying as much as possible.
Running out of lives however will bring you back to the nearest checkpoint, while resetting your score, rings, time and boost. If this happens to you, restart the stage.
Death during boss fights will take you to the beginning of the fight (and therefore brings your score, ring and boost counter back to 0) but will not reset your time. If this happens to you, restart.
ImportantIf you take too long in a stage, "Time Up!" will appear in red on the right side of the screen, near your score, and your currently achieved points will be frozen, even at the score tally at the end of the stage, locking you out of any end-of-level bonus points. It is generally recommended to restart the level if you're trying to get S Rank when seeing this as you'll miss out on too many bonus points from the Results screen. The amount of time to reach "Time Up!" varies per stage; some stages trigger it at around 3 mins, some at 8 mins.
Bosses;For bosses that are pretty much just attacking an individual target on a 2D view, kill it as quickly as possible without losing your rings to achieve S rank easily. See the guides for each of these bosses on how to defeat them in less than 3 hits to get off to a good start (generally it involves using a wisp to defeat them).
For bosses that have a chase scene with a 3D view and have multiple parts to destroy, side step during the fight as much as you can until the Quick Step bonus stops being awarded (caps at 100,000), and avoid killing the boss as fast as possible. These ships have other parts which you can destroy, like guns, search lights etc, which can award 100,000 points per item. But again, if you take too long doing any of this and see "Time Up!" then restart.
Hope this helps, let me know if you have any questions about this specific boss, though the above information should cover everything well enough for you to do this easily.