Gengar is hard. I played a ditto tech with multi and that played down in one turn hurt a ton for gengar. Also just make sure you don't have enough trainers in youir opponents hand so they can one shot a gigas. Also keep one g on gigas one on palkia and one on uxie. Kill all of the claydols as soon as it comes out because once their support it gone all htey have is a gengar you have to deal wiht.
Ttar if they are smart they will try ot power up a ttar and just spread a ton. 3 shots gets them a ton of prizes. Luckily just get a fast start and don't use a lot of powers when ttar coems out. Kill the support quickly too and they will eventurally die.