I found a sanders and it says it’s an a rank, but it appears to have ridiculously low health. Any info on it or any evolutions?

The letter ranking has nothing to do with monster strength - that only shows the % chance of scoring a bonus action. 10% for rank A. 

Its evolution isn’t much stronger - its advantage is being able to stun creatures that kill it. 

These arkadions are good when you fight opponents with endless bonus attacks.