First, is he Timid? If not, put Timid.yo rate my revenge killer gengar pls
so first i maxed out speed and special attack
then i gave him sludgebomb,focus blast ,shadowball,thunderbolt
how did i do
Second, I recommend replacing Thunderbolt with a utility move such as Pain Split or Substitute. Pain Split because Gengar needs a more reliable form of recovery, and Substitute because Gengar can do a lot of damage to a team when it's behind a sub.
And lastly, what's its item? I suggest using either Life Orb or Black Sludge, for either a massive boost to its damage output, or a form of passive recovery, allowing it to survive a little bit longer.