Damage Meter

Could you provide a bit more detail about what aspect of the Super Smash damage meter you need help with?

In a broad sense, you will want to assign each character a damage variable that tracks how much damage they've taken, and then have a helper function that converts that damage into a damageFactor which determines how much it will affect the character's knockback. For example, it could look something like this (note: I don't have Unity in front of me so consider this to be somewhat pseudocode):

 private float damage;

 // First, we calculate the damage factor based on the current damage. In this example, 
 // I divide by 10, but your actual calculation might be a lot more complicated. I would
 // recommend assigning public variables to the different values in your function so
 // you can easily test how it plays with different values.
 private float getDamageFactor() {
     return this.damage / 10;
 }

 // Here, we fetch the mass of the rigidbody and multiply it by the damageFactor. This is
 // assuming that mass is what is used to distinguish a heavy character (like Bowser) from
 // a lighter character (like Sheik)
 private float getKnockback() {
     rb = GetComponent<Rigidbody>();
     float damageFactor = this.getDamageFactor();
     return rb.mass * damageFactor;
 }

You could then multiply the result of getKnockback() by the normal velocity change of a hit in order to make a character go flying more or less depending on how much damage they have taken.