This deck centers around ramping up to a Dark Feraligatr as quickly as possible and using his 3 energy, 50dmg Crushing Blow to pop most basics and start tearing through the mid game.
The concept is to buy time with Mantines and card shuffle with Cleffa until you get a Dark Feraligatr online, then Retreat Mantine/Cleffa and Gust of Wind any of your opponent's Baby or Basic Pokemon from the bench, and one-shot it with Crushing Blow. If your opponent only has high-hp basics/no baby Pokemon - use Crushing Blow with some Darkness energies for extra damage, and Energy Removal Trainer Cards to go toe-to-toe with your opponent's more powerful Pokemon by rapidly stripping their energy. Gold Berries/Full Heals to keep Dark Feraligatr in fighting form, and other cards to help facilitate card cycling/getting the Gatr online as quickly as possible.