Underfoot Adept is a Monk racial archetype for Halflings. At lvl 4, you count as 1 size larger for determining your size modifier and the max size of enemy you can trip. It goes up by 1 more size category every 4 levels, so you are counted as Gargantuan at lvl 16 (able to trip a Colossal creature, the largest size category). However, if you are Human, you can take Racial Heritage (Halfling) and take the archetype as a Medium creature rather than a Small creature so you'll hit effective gargantuan at lvl 12. Furthermore, if you have the Ki Throw feat, you can spend 1 Ki point per size difference between your normal max and the creature's actual size. For example, if you're a medium creature, you can normally trip up to large sized creatures. But, with Ki Throw, you can spend 1 Ki point to trip a huge sized creature. You can spend 2 points to be able to reposition a Huge creature with the Ki throw or to be capable of tripping a Gargantuan creature. So on and so forth. So with lvl 4 Human Underfoot Adept with Ki Throw, you can spend 1 Ki to trip a Gargantuan creature or 2 Ki to trip a Colossal creature. You can also go for Grappling since there's no size limit on that. Nothing stops a Fine creature from grappling a Colossal creature other than the massive penalty from size difference; so long as you can make the check, you can grapple, unlike Trip where, even if you could otherwise make the check, size category difference can make it outright prohibited. So coupling the tripping prowess of Underfoot Adept on a medium creature with Grapple feats... well, you get the idea.
