The thing with Gengar v LuxChomp is that Chomp can snipe everything on your bench. Bench Shield would help but defiantly not stop it. Simply Bright Look then knock out. I usually play around Gengar. I mean ok hit me for 30 with Poltergeist or 30 for Shadow Room. Ok heal off with Healing Breath. I'll just kill everything else first then kill Gengar.
I've rarely had problems playing against Gengar with LuxChomp.