My friend has a halfling rogue and wants him to hop up onto the back of a dragon or other large creature and stab it in the back or stand on it's head and konk its head with a club.
Will there be rules handling similar situation where a smaller character wants to hop onto a larger character's back and stab/beat it? Or if not specifically, what would the best way to handle such a situation? I recall the description mentioning there being called shots rules. Would that be useful?
Acrobatics check (for jumping and landing accurately), move action. The dragon would get an AOO to hit the halfling, as he is certainly moving through the threatened area, unless he has enough movement to calculate in trying to also simultaneously avoid AOO with his Acrobatics check.
Standard attack to hit it (or initiate a grapple).
If he is not grappling the dragon, I would require him in subsequent rounds to keep making acrobatics checks to stay atop the dragon, and of course the dragon could try to shake him off (I would use a Combat Maneuver, something akin to Trip or Bull Rush).
If he's trying to accomplish something special by hitting it in the head, perhaps use the Dirty Tricks Combat Maneuver (Described in the APG) to inflict a status condition on him (likely blinded, dazzled, or deafened).
This is really a GM's call thing and doesn't really need more rules other than the combat maneuvers and skills already provided by the rules.
No, no, I've never had to deal with a halfling rogue who seemed to spend half his time dancing in and out of a dragon's mouth, no....