Koga deck

The Koga package is straightforward. Koga allows you to return a Muk or a Weezing from your Active spot to the hand, Weezing's Gas Leak ability poisons the Defending Pokémon, and Muk's Venoshock increases from 70 to a hefty 120 damage if the opponent's Active Pokémon is poisoned. This is enough to take down Pikachu ex and Mewtwo ex in one hit, while not even needing to sacrifice a point in the process.

Grimer and Koffing lines are both gifted with high HP, from their Basic forms to their Stage 1 evolutions. Muk and Weezing notably survives Pikachu ex's Circle Circuit and Starmie ex's Hydro Splash. 

By playing the best stall engine in the game in Weezing-Koga, this ninja-like strategy of exploiting the Poison effect denies your opponent from getting a point by knocking out your active Muk or Weezing. This strategy is efficient and budget-friendly enough to build decks around.

Weezing Scolipede

Mythical Island's new batch of Pokémon brought the Venipede evolution line. It's very similar to the Grimer line as Scolipede's Venoshock has the exact same damage and additional effect, and is supported by its pre-evolution's poisoning attack, Whirlipede's Poison Sting.

Scolipede's Venoshock attack requires one less than Muk's, and its entire evolution line boasts a more manageable retreat cost, making it a natural replacement for the Gen 1 Pokémon despite needing one extra evolution. Both Venipede and Whirlipede only require Energy to attack, as do Koffing and Weezing. This aggressive Energy efficiency allows the deck to overwhelm opponents that rely on setup, particularly those it holds a type advantage over such as Mewtwo ex.

Koga Muk

While Grimer has a serviceable attack that helps set up Muk, it's not an ideal opener. Since Koga returns all attached cards to the hand, including pre-evos, Grimer can be played as a one-of (technically up to three with Poké ball's effect) while Muk can be kept as a two-of to maximize your chances of evolving Grimer at the right time.

The biggest line is to use Gas Leak, then activate Koga on Weezing and attack with Muk's Venoshock for increased damage. Sure, it's situational, but this combo can take down most Pokémon in the game and is ideal to get an easy point on an unprepared Pokémon.

Tech cards in Koga decks

Salandit (Mythical Island)

Salandit brings its own Venoshock attack to the table, going from a base 10 damage to 50 damage if the Defending Pokémon is Poisoned. What sets Salandit apart from other Venoshock users is that it doesn’t need to evolve and only requires 1 Energy: a Energy that makes it easily splashable.

It can be played in a mono shell while still hitting for more weaknesses, against most and type Pokémon.

Weaknesses

• All the main Pokémon, except Koffing, are plagued by a high Retreat Cost. While Koga's effect helps mitigate this, it uses up your Supporter card for the turn. Grimer, on top of a 3 retreat cost, cannot be brought back to the hand using Koga.
• Muk's attack is also quite expensive. As a result, you cannot smoothly transition from a turn 1 Grimer to a turn 2 Muk ready to use Venoshock. This is a huge loss in tempo that no Energy Acceleration effect can salvage: there are none for types in Genetic Apex. This is why Arbok is considered a better partner for Weezing currently.
• For lack of a Pokémon ex, and if you haven't set up Poison before attacking with Muk, the overall damage output of the deck is mediocre.