Baby Box Control

Setup:

The setup is simple and easy. Go second. Buddy poffin for 2 dunsparce and Telepathic energy on Elgyem for Clefairy and Latias. This differs for each matchup, so check the meta matchups you could have below.

Matchups:
Festival Lead: Very favourable matchup. Make note that their decklist plays 5 energy. Using Elgyem to move 5 energies is enough for you to win the game. You could also Clefairy the Thwackey (since it has 2 retreat)
Dragapult (pure): Benching anything eg. Bloodmoon, Lillie's Clefairy,

Setup:

The setup is simple and easy. Go second. Buddy poffin for 2 dunsparce and Telepathic energy on Elgyem for Clefairy and Latias. This differs for each matchup, so check the meta matchups you could have below.

Matchups:
Festival Lead: Very favourable matchup. Make note that their decklist plays 5 energy. Using Elgyem to move 5 energies is enough for you to win the game. You could also Clefairy the Thwackey (since it has 2 retreat)
Dragapult (pure): Benching anything eg. Bloodmoon, Lillie's Clefairy, Meowth will lead to very favourable outcomes. In the case of Bloodmoon, focus all of your resources on using Clefairy, and use Ruffian to remove their Air Balloon. Remember that the game is not lost if they bench Latias. A Sudowoodo can fully prevent their retreat. Battle Cage is incredibly important in any Pult matchup. Make sure you use it when you think they do not have Jamming Tower in hand.
Dragapult (area zero): Focus on using Elgyem off the bat. They will be playing Unfair Stamp as their Acespec, not Neo Upper. They will have a heavy reliance on Crispin, so Elgyem will be practically useless if you cannot chain it (only keep using Elgyem if you can consistently use it). In other situations, you will have to use Sudowoodo/Clefairy to win the game
Crustle: This matchup is slightly harder. We need to force them to bench as many Pokemon as possible and Elgyem will win us the game.

Rocket's Mewtwo: This is a more normal matchup. Elgyem will do exactly as it should and you should score a game win. Cornerstone can be powerful but you must be careful of Mimikyu. Ensure you have used Elgyem at least twice or if they have any Rocket's energy in the discard pile before using it (since they can easily chain Mimikyu twice), In this matchup, you must note that they have Giovanni, allowing them to easily get out of retreat lock (retreat locks/clefairy are less powerful)

Cynthia's Garchomp: This matchup is especially hard to win. This is because Garchomp has a retreat cost of 0, low attack cost, and the deck plays multiple energy. Our only hope is gusting a forcefully benched Roselia/Roserade

Ogerpon-Meganium: This matchup is also especially hard. Since this deck focuses on energy disruption, Meganium's ability makes it almost impossible for them to run out. Your Cornerpon is basically useless against this deck since (if) they also have Arboliva. However, if they do not play Arboliva, Cornerpon instantly seals the game. 

Raging Bolt: Against Raging Bolt, it is essential to stay alive for as long as you can. After you use Clefairy (Elgyem doesn't do much in this matchup), your opponent will likely bench a Latias. You must stall for as long as you can to wait until your opponent has only 11 cards or less in their deck (you can also cycle Rocket's Larvitar). Then, you can gust the Latias to retreat lock it. You can also stall for more turns by Gusting other Pokemon such as Noctowl and Meowth.

Alakazam:
No question. Set up Rocket's Larvitar and Rocket's Articuno immediately and start milling with Larvitar. Your Larvitar will be unscratchable and you will score and instant win.