A character can climb on the back of a creature at least two size categories larger, he gains +2 on attack rolls, the creature is considered flat-footed against your attacks.
Initiating a Climbing Attack
Initiating a climbing attack on a creature is a special full-round action. In order to perform this maneuver, a you enter a creature’s square (potentially provoking an attack of opportunity for the movement, unless the character makes a successful Acrobatics check). Once the movement has been made, you then makes a climb check against the target's Combat Maneuver Defense. Characters with a climb speed gain a +10 bonus on this check. At the end of this full-round action you may male one melee or ranged attack at your highest attack bonus.
Maintaining the Climbing Attack
Each round you remain on the creature, you must make an acrobatics or climb check (your choice) to remain atop. A creature may spend a full-round action on its turn trying to shake you off, forcing you to immediately make an acrobatics or climb check as if you began the round on the creature.
In either case, you must make Balance checks opposed by the creature’s Strength checks on both your turn and your opponent’s turn to keep your footing. While clinging to a creature, you cannot take a full-attack action, and if you are knocked off, you fall prone in an adjacent square (use the grenade-like weapons table to determine the direction you fall in).
The Bigger They Are (Combat)
Prerequisite: Climb 4 or Acrobatics 4
Benefit: The targets of your combat climb no do not get their size bonus to their combat maneuver defense. When performing a combat climb, you may maintain the maneuver as a swift action.
The Harder They Fall (Combat)
Prerequisite: The Bigger They Are, Base Attack Bonus +12
Benefit: When performing a combat climb on a creature, you automatically confirm critical threats with attacks made against them.