0 Atk Mega Sableye Knock Off (97.5 BP) vs. 0 HP / 0 Def Gengar: 276-326 (106.5 - 125.8%) -- guaranteed OHKOhttp://replay.pokemonshowdown.com/ou-371704559
The damage calculator you Smogon people use is obviously flawed. First turn Gengar died to a hit Gengar doesn't die to. Change the rules to Lv. 50 only like how it is ingame too, it will make for a much more genuine battling experience.
I then changed the levels of both Pokemon to Level 50.
0 Atk Mega Sableye Knock Off (97.5 BP) vs. 0 HP / 0 Def Gengar: 146-174 (108.9 - 129.8%) -- guaranteed OHKO
In case you were wondering, yes even 0- Mega Sableye's Knock Off will OHKO after Life Orb damage.