Gengar very viably run a Choice scarf set.
However, there are several simpler ways to get rid of the opposing Greninja, if that's what you're afraid of:
-->Mega Gengar with full speed investment outspeeds a Greninja at full investment and provided it does not have a Focus Sash or a Scarf of it's own, you'll almost definitely win the odds.
> 252 SpA Mega Gengar Sludge Bomb vs. 4 HP / 0 SpD Greninja: 238-282
> (83.2 - 98.6%) -- guaranteed 2HKO
Even at max investment in Special Attack, a Greninja cannot KO a Mega Gengar at max HP.
--> Focus Sash is another great choice for an item, if you don't want to get revenge killed by a Greninja, although this set will lose to a greninja at full health, so Stealth Rocks/Spikes or prior damage can also be helpful.
Other than these, Sticky Web also secures you the first strike, so it should also be considered.
Hope I helped!