4 attacks LO Gengar is surpirsingly effective, especially in Volt-turn teams, where its frailty is not a big issue. Shadow Ball + Focus Blast are musts, where on the other two slots you have three options: Sludge Wave is nice for Fairy-types, Mega Venusaur (Sludge Wave 2HKOes defensive Mega Venusaur 80% of the time with SR), Greninja (on the switch obviously, as nobody likes to spam Focus Blast on forced switches), Mandibuzz (70% chance to 2HKO without SR), Assault Vest Tangrowth, and can in general OHKO a lot of offensive Pokemon that Shadow Ball can't. Thunderbolt is great for Mega Charizard Y (doing 77% minimum to this beast is very useful), Togekiss, Skarmory (OHKOes after SR, unlike Shadow Ball, which is useful in HO teams that want to prevent Skarmory from using Defog), Keldeo, Mandibuzz, and regular Gyarados. Finally, HP Ice OHKOes Dragonite after SR, Garchomp, Zygarde, Gliscor, and Landorus-T. Imo, the best comob for the last two slots is Sludge Wave + HP Ice, which allows you to OHKO / revenge kill a ton of offenisve Pokemon, while simultaneously being very hard to wall, as many teams use Fairy-types as their special wall, or Assault Vest users as their special pivots on offenisve teams, all of which Gengar can 2HKO. HP Fire is also an option, but all the Steel-types you would want it for are already 2HKOed at worst by Shadow Ball, so it's usefulness is very limited.