252 SpA Starmie Psyshock vs. 80 HP / 0 Def Mega Gengar: 278-330 (98.93 - 117.43%) -- 93.75% chance to OHKO -- guaranteed OHKO after Stealth Rock.
I'm pretty sure you meant EV's, not IV's, so...that's what I went with. Gengar's chances of survival are...very slim, sadly.
Yes, I did mean Ev's lol. Haha Starmie won't be getting past me anytime. Tho if stealth rocks are not up, I think MGengar will shadowball it to death. Also, those stats are asuming Starmie was sent in first, so not many people will risk spinblocking with MGengar.