It is NOT a great guide.
I made myself a list of monster (as 28 March 07) in Hero.
I found out that, the amount of experience that you get is base on the monster level (beside you got the experience +)
I would suggest you try to plan kill the monster few levels upper than you.
Before you select the monster, check its health and attack power!
If we want to kill a level 37 monster (Scorpion and Green Goblin), we checked their health!
Scorpion is having 522 and Green Goblin is 806.
We choose Scorpion to kill!