Giantbane( Complete Warrior, p. 111)
[Fighter Bonus Feat, Tactical]
You are trained in fighting foes larger than you are.
Tumble 5 ranks, base attack bonus +6, Medium or smaller size,
The Giantbane feat enables the use of three tactical maneuvers. Duck Underneath: To use this maneuver, you must have taken a total defense action, then have been attacked by a foe at least two size categories larger than you. You gain a +4 dodge bonus to your Armor Class, which stacks with the bonus for total defense. If that foe misses you, on your next turn, as a free action, you may make a DC 15 Tumble check. If the check succeeds, you move immediately to any unoccupied square on the opposite side of the foe (having successfully ducked underneath your foe). If there is no unoccupied square on the opposite side of the foe or you fail the Tumble check, you remain in the square you are in and have failed to duck underneath your foe. Death from Below: To use this maneuver, you must have successfully used the duck underneath maneuver. You may make an immediate single attack against the foe you ducked underneath. That foe is treated as fl at-footed, and you gain a +4 bonus on your attack roll. Climb Aboard: To use this maneuver, you must move adjacent to a foe at least two size categories larger than you. In the following round, you may make a DC 10 Climb check as a free action to clamber onto the creature's back or limbs (you move into one of the squares the creature occupies). The creature you're standing on takes a -4 penalty on attack rolls against you, because it can strike at you only awkwardly. If the creature moves during its action, you move along with it. The creature can try to shake you off by making a grapple check opposed by your Climb check. If the creature succeeds, you wind up in a random adjacent square.
A fighter may select Giantbane as one of his fighter bonus feats.