The SinX MVP has about 1.4 mil HP and 62 VIT.
Providing that you have Djinn, Kahos, Celebs and job 70 you'll get around 37 bonus INT. You'll be doing around 40k per AD. Which means that after 32 AD's yuo have done a bit over1.3 mil worth of damage. So yes it's completely normal. Also providing that you didn't get rid of the MVP slaves it's veryu likely that the SinX walked into a Sanc that the HP casted and thus got some of the HP back.
And I really doubt that you one hitted the SinX MVP unless you hack.