You scramble onto the creature’s back, distracting it and gaining a better position to bring it down.
Move Action Melee 1
Target: One Large or larger creature
Effect: You move into the target’s space, provoking opportunity attacks as normal. Until you leave the target’s space, the target grants combat advantage to you and your allies and takes a –2 penalty to attack rolls, and you grant combat advantage to other enemies. Any attack that damages you also deals half the attack’s damage to the target. When the target moves, you move with it, staying in the same portion of the target’s space.
If the target hits you with a melee attack, it can slide you 1 square to a square adjacent to its space unless you succeed on a saving throw.
Special: If you’re trained in Athletics, you don’t grant combat advantage to other enemies because of being in the target’s space, and you can make a saving throw to negate any pull, push, or slide that would move you out of the target’s space.
....Now I've been trying to do this for years. Some DM's let you do stuff like this with an extremely difficult series of rolls, most just shake their head and chuckle. You see, I always play Dwarves, and any self-respecting son of Moradin knows that the best place from which to smite a large foe is from the top down. Thank you WOTC.
I just wish this was an At-Will.