Rechan said:"Okay, I want to run up the monster's tail, leap on it's back, and begin shooting it in the back of the head."
Rules Heavy system. "The monster is going to get an attack against you because you entered its space. Going up its back is difficult terrain, meaning that you can only get half way up its hind quarter. Also, you need to make a balance check to succeed in the first place. Next round, you make a balance check at a higher DC just to stay on as it starts to fight back."
Rules Light system: "Okay. In fact, I'll give you a +1 for doing that."
Rules Heavy system #2 "You can't do that, running up a monster is not covered in the rules"
Rules Heavy system #3 "You can't do that, there's a feat in Dungeonscape that lets you climb a monster, but you have to attack it with a hammer and a spike first"