Harder They Fall
You can work with an ally to move or knock over a foe that's too large for either of you to overcome alone.
Prerequisite: Str i3, Int i3, Combat Expertise, Power Attack.
Benefit: When you use aid another to grant an ally who also has this feat a +2 bonus on the attack roll for a bull rush or trip combat maneuver, the ally can attempt that maneuver even against foes two or more size categories larger than herself.
Source: Giant Hunter's Handbook, p. 20