Credit goes to
Hack He Must for the following set (I'm not entirely sure about the EVs):
Gengar @ Gengarite
Ability: Levitate
EVs: 252 HP / 176 Spd / 80 SDef
Timid Nature
- Perish Song
- Protect
- Disable
- Hypnosis / Taunt / Substitute
Perish Song + Shadow Tag removes any Pokemon of choice, as long as Gengar can survive against it. This set is downright unfair against stall, as stall relies on the collective synergy and walling power of its team, and removing a key component can open a gaping hole for an offensive threat to take advantage of. Moreover, many of the mons used on stall struggle to hurt this set, as they rarely carry multiple powerful attacking moves, and at least one of their moves can always be disabled, leaving them unable to significantly damage Gengar.
Perish Song is the point of the set, allowing Gengar to remove anything that fails to KO or phaze it in time. Protect allows Gengar to Mega Evolve unhampered, and Protect + Disable goes a long way in helping Gengar avoid damage, paralysis, taunts or anything else that would hamper it. Hypnosis may seem like a terrible idea with 60% accuracy, but against stall mons which can usually only phaze or status Gengar, risking a miss is not particularly devastating. It can also help against offensive mons, which the set otherwise struggles with, but is unreliable. Taunt further helps to shut down stall mons against it and more reliably prevents phazing (which now works through Protect), but is less generally useful. Substitute helps against offensive mons, but is costly due to the complete lack of recovery.
Although I emphasized how well this set performs against stall, it also performs against mons other than walls. Most Choice Band/Specs mons (unless they outspeed Gengar, which is unlikely) are easily eliminated as Disable reduces them to using Struggle. Support mons and some offensive mons also only carry 1 attack that can hurt Gengar, allowing it to shut them down and eliminate them.
The EVs allow it to outspeed Arceus formes, while it struggles to beat Darkrai either way and Shaymin-S are often Scarfed. Remaining EVs maximize its Special bulk (physical bulk is a lost cause, not to mention that physical priority is far more common and powerful priority threatens this set).