With proper dodging he's an ideal tree killer. The poison typing eats through the grass type of trees and ghost type eats through his psychic typing. You just to dodge some fast attacks and really dodge if it has psychic charge. He defends well against the grass charges the tree can have. Dodging nullifies a lot of the damage so he's rated highly as long as you aren't face tanking.
GamePress
Use for Gengar?
Gengar is now a high tier attacker, but what would you use him for? Attackers have the option to select counters, and I don't really know anything that Gengar would do well against, his poison typing kind of screws him over. And if I was just gonna ignore typing and use a neutral attacker, why not just use vaporean etc? I have a good Gengar but can't justify investing in him - why is he high tier?