You can easily get past Gengar using either your Feraligatr or your Crobat. Pick one of them, and make sure it's faster than Karen's Gengar, isn't holding a choice scarf, knows screech, and knows a strong dark type attack (crunch for Feraligatr and bite for Crobat). When fighting Karen's Gengar, use screech until it uses a move other than destiny bond, and then use your dark type attack. This works because destiny bond only lasts until the user uses a different move.
