Earth Child Style
Twin Thunders Flurry
target must have the giant subtype
Underfoot Adept
Halfing
Ki Throw
Spend Ki
Powerful Shape
This is a little sketchy, but considering it adjusts CMB, I don't see why it shouldn't count. Worst case scenario, Beast Shape lists Trip as one of the SQ you can gain, so it would affect that for those forms.
I'm fairly certain that spell effects that offer trip checks don't have a size restriction. This includes Toppling Spell and Hydraulic Maneuver
Seems like that may not be everything, maybe there's a specific weapon or something, but I'm not finding/remembering anything else atm.
Halfling Monk is probably the most reliable in terms of being able to apply the bonus(and have it improve), followed by Powerful Shape.
