Despite the large variety of cards available, only a few strategies make up what is generally considered the highest "tier" in Unlimited play, with most competitive decks being either "donk" decks focused on winning by KOing your opponent's only Pokémon in play, or "lock" decks focused on preventing the opponent from taking meaningful actions.
Forretress Donk is designed to win on the very first turn of the game (a Donk City) by taking all prize cards and/or knocking out all of the opponent's Pokémon.
Shiftry Donk is designed to win on the very first turn of the game by shuffling the opponent's Pokémon from play to their deck with Shiftry's Giant Fan Ability, fulfilling a possible win condition.
The term “donk” refers to when a player wins the game on their first turn by removing all of the opponent’s Pokemon from play. While this sometimes happens due to bad luck, there are some decks that build an entire strategy around the concept of pulling off the donk.
"Donk decks" are those decks that focus on winning round 1 by removing all cards from play. For example, decks like Forretress donk, Shiftry donk are design to win on first turn using some kind of strategy. Since they are incredibly luck based and unfun, many of them have been banned, and only usable in Unlimited format.
Hope this helps!