Terminal Velocity is different from other zones in which there's simply three stages, none of which have red rings of them, and only two of which grade you for your score.
I recommend not attempting this until you have Super Sonic for an easier time, though you are free to challenge yourself without him.
The whole stage is just Sonic automatically running forwards, while you need to dodge the lasers and other obstacles that the big bad is firing at you. Keep a close eye on its arms to see where the lasers are going to fire and dodge accordingly.
If you have Super Sonic, you can easily collect 50 rings from the first section before the big bad appears, then press RB to transform to Super Sonic and boost all the way to the end of the level to achieve an easy S rank.
This time you're fighting the big bad head on. To achieve enough points for S rank you'll need to defeat him in less than 3 minutes, without losing your rings - sounds daunting but it's fairly straight forward once you know how.
The battle goes from you running towards him for 1 attack until he's exposed, running away from him for two attacks until he's exposed, to you running towards him for two attacks until his exposed to you running away from him for three attacks until he's exposed etc etc.
When he's exposed, his arms will flail around and you can target him with your homing attack. Use the homing attack on its hand, shoulder, top of its head, enter slow-mo, attack face. Slam down with X and boost immediately after to collect the rings dropped from him and to catch up again, then jump when you're right next to him to give the final kick.
You should only have to do this 5 times before your final 'wisp attack' triggers, to which you press RB followed by a homing attack to finish off.
When running towards the boss, there are three types of attacks you'll need to be prepared for, which increase in difficulty (through faster moves or more attacks) the longer you spend in the fight;
They come towards you at varying patterns, side step left and right to dodge them. These increase in length, but not complexity as the fight goes on.
His easiest attack to avoid.
Rows of purple orbs will come towards you which require jumping over to avoid. I recommend jumping and then pressing X to slam down immediately after making the jump, so that you land quick enough to jump the next row. Initially starts with two rows, and increases by one gradually as the fight goes on. I believe they might also increase in speed as the fight goes on as well.
This attack is easy to avoid once you familiarise yourself with the jump/slam rhythm.
He will fire a laser sweeping the path from right -> left and/or left -> right and alternate between the two directions. Initially starts with three 'sweeps' and adds two more sweeps the attack that also increase in speed as the fight goes on.
This attack can catch you out as it speeds up if you're not careful. Stay in the middle lane during this attack and remember you can double jump and boost if it looks like you'll land too early.
This attack is one of the harder ones to avoid.
When running away from the boss, you will have the above attacks to dodge as well as two fresh ones - I found these harder to dodge since it was harder to tell what exactly was coming, though with a careful eye you can still differentiate;
Black cubes with purple orb
If the black cubes land in two sort of L shaped patterns, then expect a purple orb to roll through a set path and jump over the orb to avoid. If you can navigate through the pattern using up/down to speed up/slow down and left/right to sidestep into one of the corners where the orb does not roll through, you'll be safe. Once three orbs pass through, navigate out of the maze before the cubes shoot off, catching you with it.
This attack is not too challenging to avoid once you recognise the pattern that lands. Typically if I see him firing cubes, I'll immediately start boosting so that I'm far in front of it all giving me more of a chance to better analyse which attack it is.
Black cubes with purple laser
If the black cubes land in a sort of zig zaggy pattern or a three point triangle, expect a purple laser to shoot through each of the cubes. Quickly spot a safe zone and stay there until the laser fades.
This is the hardest attack to avoid due to the random nature of it. You'll unfortunately just need to try and spot the route of the laser before its fired and jump/double jump/boost to try and avoid it. There's also an element of luck to this since it depends where the cubes land around you...
I finished the fight with around 160 rings and in just under 3 mins. I maxed out my sidestep score (100k) as much as I could after each of my attacks while the big bad was flopping around and achieved +4mil points leading to an S rank. Super Sonic is not allowed during this fight unfortunately, so you do have to do it properly. Practice practice practice and you'll eventually push through. Good luck!
This stage is ungraded. Just run through and dodge some of the traps thrown at you. In the section where the floor starts coming up, STOP SIDESTEPPING or you'll risk falling into one of the pits. Just keep running forwards and boost through to the end.