Ranks in Ex-Zodiac are based on your point score from the mission. The ranks you receive after each mission are based on point thresholds you have to clear. You can gain points in 4 different ways:

1. Level Score
   
2. Enemy Bonus
   
3. Min. Damage Bonus
   
4. Boss Time Bonus

Your level score comes from shooting down enemies. Each destroyed enemy adds to a combo which lasts a few seconds. Your combo works using the following exponential formula
Hits x (100 x Hits)

Although they are less common, there are some enemies that will add more than a 100 points to your combo when killed, however those only count as a single hit.

This bonus is awarded if you have destroyed every single enemy in the mission. Getting close will still award points, although, much much less.

This bonus is awarded if you survive the mission with as little damage as possible. Receiving enough damage to down your ship counts as 100%. Resetting at a checkpoint will not be counted.

This bonus is awarded based on the time it takes to defeat the boss at the end of each mission. I could not find a reliable source for the formula for this calculation, but it seems to use a large number and then exponentially decrease the score based on your time.

This section is a general list of helpful tips that I found useful when attempting to achieve S ranks.

• You are not timed during the mission, be sure to make use of your brake.
  
  • Braking allows you extra time to destroy all the enemies on screen.
    
  • Braking can make navigating tricky obstacles easier.
  
  
• Combos are extremely important for high scores.
  
  • Try to delay killing enemies as long as safely possible to help string together groups of enemies.
    
  • Simply hitting an enemy will refresh your combo timer. Try not to immediately destroy the more tanky enemies as killing them slowly will allow you to keep your combo up longer.
    
  • While you should use your lock-on micro missiles to guarantee risky shots, they do take longer to fire and hit. If you're trying to save a combo, consider firing a standard laser.
    
  • Flying enemies keep pace with you while ground enemies do not. You can exploit this by boosting flying enemies into the gaps in ground enemies where your combo would normally die.
  
  
• Use bombs as often as you can to clear out large enemy groups or to save your combo.
  
  • It should be noted that while bombs are exceptionally helpful, the timing on them can be rather weird. Try to have designated spots in levels where you use them, rather than improvising to minimize their variability.
    
  • Certain bosses are more susceptible to bombs than others. Be sure to plan ahead and use bombs wisely. (or just abuse the checkpoints)
  
  
• Don't forget you have barrel rolls and that they deflect incoming fire.
  
  • No seriously, don't forget.
    
  • This is incredibly important if you want to avoid damage.
  
  
• Restarting from a checkpoint by dying in the middle of the stage will reset you to your default configuration.
  
  • This can be helpful to refill your bombs at different intervals on the stage.
    
  • This will also remove any laser upgrades you may have acquired.
    
  • It will also give you a second shot at getting all the enemies, although restarting the stage is probably better.

I ended up flubbin the combo in the recording, but the robots that push down the windmills at the end are worth a lot of points and should be combo'ed.

This mission is relatively forgiving and an S rank shouldn't give you too much trouble as long as you remember to take your time on the large enemy groups to preserve your combo.

There are two main combos here. The one right at the start and the big one near the tail end. The first one can be a bit tricky but has a huge payout. The entire end section can probably be combo'ed all in one go, but this works.

Be sure to save your bombs for the boss as it can take a while if you don't burst it.

This mission is rather easy to cheese with checkpoint exploits. You can destroy the brown rocks to gain points, but they do not help your combo.

Be sure to save your bombs for the satellite enemies and then reset to checkpoint at the boss to burst them down with bombs.

This mission is a little tricky. Most of your combos can be pretty loose without any real downsides.

The important bit is getting all the sandworms with a bomb. Each section of the worms count as a hit, but they're usually underground. A well place bomb should get you a monster score.

The boss has a lot of HP so be sure you grab lasers by flying through the rings at the start.

Most of your time in this mission will be spent attempting to keep up with all the enemies that spawn. Lock-on missiles are super helpful here. Try to use bombs to decimate large groups of the orange fish.

You can shoot the mines to gain combo, and you can shoot the large flying creatures to maintain your combo.

Braking is super important in this mission. Try to keep the enemies in front of you to avoid missing out on extra points. Try to use the civilian vehicles to build up/maintain your combo.

Combo'ing the bipedal enemies is exceptionally valuable since they give 600 points each.

Once I get around to checking out the new content, this'll be updated! Thank you for your patience.