I was doing some research on monsters and i found that a monster that is exactly 24 levels higher than your current level will give you the max amount of exp. All monsters that are higher or lower will not give you this max exp. I tried this on my hawk archer, Koyagichan on Apoline level 36 at the time, and my friend Mordag, who is a level 26 clever fighter on Apoline. Both of us got our golden ratio at monsters that are 24 levels higher than we are. Mind you this is for all regular monsters. Elites give more exp than regular spawning monsters, but they take a lot more time to kill.
Help on our quest of knowledge goes to Faia , level 50 High Cleric and also on Apoline.
I also found a nifty site showing all the monster levels 1- 100 on this site.
http://www.fiestaonlinehq.com/index.php?pid=173