The higher the ranking the more resistant a monster is to a certain type of attack
S rank being the highest and guarantees 100% damage resist (except in a few cases)
D rank is the lowest, meaning no resistance capabilities.
Only speed is capable of reaching E rank.
Now to explain why S rank doesn't block 100% in certain cases.
Let's take a look at Ghost:
S rank Melee
A rank Missile
D rank Magic
Ghost is weak against magic and strong against Melee and Missile. However any unit above 3
can damage ghost with melee, provided they have air attack. This is because of the rank difference.
Now take a look at Succubus
C rank melee
C rank missile
S rank magic
This monster is completely unaffected by magic, I've personally went up against her several times, and all damage from magic attacks go to zero.
Other thing is, what's the type for the monster? ground, air or sea?
I'm not entirely sure how much what affects what. All I know so far is air units have resistance against ground. Sea is generally weaker against magic attacks.
That's all I know so far .3.' sorry for long post