Low Blow

Mobility (PH) , base attack bonus +4,

As a full-round action, you can enter an area occupied by an opponent who is at least one size category larger than you. You can then make a single melee attack at your highest attack bonus against this creature, who is considered flat-footed against the attack. After your attack, you return to the 5-foot square from which you entered the opponent’s square. Using this feat provokes attacks of opportunity.

You can get underfoot and attack creatures larger than you.