Ok here's a look at the possibilities: Espeon's Yawn has the accuracy of 100% while Gengar's Hypnosis has the accuracy of 60%. Yawn takes two turns before activation, so a switch out may occur, while with Gengar you MAY miss and risk fainting hhim rendering your strategy a fail.
1. Use this strategy in a double battle.
2. Use Gengar's Dream Eater w/ Mega Evolution (For extra power) + Amoongus'/ Breloom's Spore for a 100% chance. (But watch out for the ability Overcoat, other Grass types, and also Safety Goggles can nullify Spore) Breloom would be better offensively than Amoongus, but here's a strategy you could use. :)
Hope I've Helped