The Pain Split set is akin to the Disable set, but the approach to this set is much more offensive. Pain Split acts like a third attacking move capable of muscling through typical counters with massive HP stats such as Blissey, Chansey, and Snorlax. Furthermore, Pain Split works perfectly in conjunction with Substitute and Life Orb, and even benefits from the residual damage from sandstorm and hail. The closer Gengar's health bar gets to zero, the more HP Gengar can suck away from its opponents. Gengar's coverage is impressive, and it can act as a fail-safe sweeper at any given time even with the absence of an extra coverage move.
The combination of Substitute and Pain Split allows Gengar to take out Pokemon it normally would not be able to, most notably Blissey. After Substitute and two Pain Splits (or one if sandstorm is in effect), Blissey's health will be low enough that Focus Blast 2HKOes. This means that Blissey will be forced to constantly heal herself or risk being KOed. Pain Split has 32 PP, while Wish and Softboiled have 16, meaning that Gengar can easily stall Blissey out. Finally, most Blissey are unable to break Gengar's Substitutes, meaning Gengar can usually come out of the duel unscathed.
Toxic Spikes is the most beneficial form of entry hazard to use with this set in order to hasten the damaging process against the likes of Blissey. Forretress makes an excellent supporter who can provide all types of hazards, especially Toxic Spikes. Ferrothorn makes a great partner, able to open up opportunities for Gengar to switch in, in addition to providing entry hazards. Heatran should also be considered as an offensive partner due to its ability to draw in more opportunities for Gengar to switch in. Gengar has such atrocious defenses that it's actually quite difficult to pull off strategies like this, so always consider Pokemon that draw in Normal- and Ground-type attacks for Gengar to switch in on.
With such a direct approach to this set, Gengar is highly capable of beating many special walls with some good timing. As a result, it is recommended that Gengar is paired with sweepers who would normally have a difficult time bypassing said walls. Life Orb Latios, Volcarona, and Choice Specs Politoed make fine offensive partners who are all capable of sweeping after these special walls are eliminated.