In Xiao's favor, like the good Captain said, the stronger she gets, the less she needs a weapon... But given her poor weapon selection the only weapon that would be better for her than her 3☆L1 Passive would be her own TMR. At 6☆100, she has 131 ATK... Which means that if she goes unarmed, she's getting a bonus of 65 ATK right now (up to 78 with maximum pot increases), which dwarfs every other fist in the game. The only other fist that would be useful for her at max level would be the Winged Saint from the FF13 event, just for the defense bonuses.
That said, her ideal non-TMR equipment loadout would probably look something like Non/Non/Tiger Mask (for more ATK) or Green Beret (for more DEF/HP)/Black Belt Gi/Hero Ring/Hero Ring. That would put her at around... What, 306 ATK? Add in 4x ATK +10%s, and that brings her to 358, and then Ifrit is another 52ish ATK, so... 410 ATK with non-TMR stuff?
EDIT: Also finally finished out the fights for the Great Nian Beast Attacks. Missions: Defeat Nian with a Limit Burst, Defeat with an Esper, and Defeat with an Item. I'm just happy I managed to get all three in one go.