This section covers some cards (usually played as 1-ofs) which target certain Pokémon.
These target the Weakness of opposing Pokémon in order to make favorable trades. Picture this: your opponent spends a search Supporter, or even a Rare Candy, to get their big scary Evolved attacker out. You bring in your Mew, which is just a Basic, Copy its attack, and KO it. Maybe said attacker was a Pokémon-ex, so you don't even care if they revenge kill Mew, as you're up 2 Prizes to their 1.
You will likely need rainbow Energy (Holon's Castform/Magneton/Electrode, or δ Rainbow Energy for the Mews) to pay for the copied attack.
Setup attacks can be copied too, like Holon's Castform's Delta Draw (which does count Mew δ/Mew ☆ δ itself), as can annoying disruption like Stantler's Push Away. Against Camerupt, if you play a lot of basic Energy, you can copy its Back Burner to undo the Energy discard from Numel's Kindle.
Que has a very detailed article on common copy targets.
These are capable of 1HKOing Dark Tyranitar, the namesake of one of the top decks in the format. However, the Dark Tyranitar deck can counter these by playing Holon Energy FF.
Regirock ex requires double Fighting for Tonnage, so you will need Holon's Pokémon to fix colors. Additional support which would be good to have are Energy movement (Regirock ex is susceptible to getting Pow-ed out and KOed by Bite Off first; with Energy movement you do not need to commit to it right away) and switch cards (for its huge 3 Retreat Cost).
With Hitmontop, you need at least 20 damage on the Dark Tyranitar before Upward Kick 1HKOs it. Decks that can spread damage, like Camler or Banette ex, can find use for it.
These hit Pokémon that tend to have a lot of Energy attached to them as part of their decks' core strategies:
Exeggutor (for Weakness; 4 Energy on the Exeggutor is enough for Psywave/Psychic Boom to 1HKO)
Delcatty
Rayquaza δ
Some Evolution lines are blessed with a Psywave/Psychic Boom attacker on the Basic stage. Gardevoir is one of them, Grumpig is another less common one.
Crystal Shard changes your Pokémon's type to Colorless, letting it hit Pokémon that are Dragon-types (in the video games) for Weakness.
Dark Dragonite is the most common Colorless-weak Pokémon. It resists Grass and Fighting, so those types of decks may find Crystal Shard worthwhile. Dark Dragonite decks often pack Mr. Briney's Compassion, so Crystal Shard is much more effective if your attacks can do at least 60 damage for the KO.
A related, but much rarer-seen tech is Zangoose, which can 1HKO Dark Dragonite for 3 Colorless Energy with Super Slash. Who would've thought that Zangoose's claim to fame would be destroying Dragonite instead of Seviper?