Were you using hacks in our battle?
Well, I just ran a damage calculator and based on my findings, a Night Slash coming from my Weavile should have done around 40% damage to your Uxie. That's based on if your Uxie is +Def natured, and has 252 EV's in HP and DEF (Also assuming perfect IV's)
When I did use Night Slash however, it seemed like it did very little damage, around 15%. I've saved the battle video and am ready to take this to the Blacklist thread...
Could you explain yourself?