...you don't want to know.
Okay fine. But it's quite simple, and HORRIBLE.
You remember the Exemplar? Well, the main thing here is that the class has an ability where you can make any skill check and it counts as a diplomacy check. Also, a DC 80 Escape Artist check lets this happen:
Okay, now you might get where I'm going with this, but it gets worse. Check out this line from Righteous Might:
If insufficient room is available for the desired growth, you attain the maximum possible size and may make a Strength check (using your increased Strength) to burst any enclosures in the process.
The burst DC of leather (which is stronger than flesh) is rather low, so the DC is quite low.
Put it all together in a character with the ability to make that DC 80 Escape Artist check, and you get a halfling who squeezes his entire body into a target's arse, instantly causing the target and everyone around to become a fanatical follower of the halfling. Then the Halfling, using Still Spell, casts Righteous Might and explodes the guy from within, resulting in cheers from his new minions. And now the Halfling strikes a heroic pose and flashes a big smile, cause you gotta land the dismount.
Best way to kill a BBEG ever.
...what? You asked.
JaronK