Monster Cards and Bestiary

What uses are monster cards for?
And also, how does one know weaknesses of certain enemy from Bestiary? I'm confused with "Very Strong, Bit Strong, Bit Weak, Very Weak" description. I simply dunno which one is weakness and which one it have good resistance to @_@
And also, how does one know weaknesses of certain enemy from Bestiary? I'm confused with "Very Strong, Bit Strong, Bit Weak, Very Weak" description. I simply dunno which one is weakness and which one it have good resistance to @_@