"How Many Trainers Do I Need to Beat Gigantamax Gengar in Pokémon GO?"
There were numerous complaints last weekend about the difficulty of completing Gigantamax Battles during their debut weekend. As a result, Niantic made some adjustments to both the difficulty of taking down Gigantamax Gengar and the rewards you receive afterwards.
Unfortunately, you will still require 20 - 25 Trainers with very good counters to defeat Gigantamax Gengar in Pokémon GO.
It is EXTREMELY important to note that the difficulty of taking down Gigantamax Gengar depends on what move it has. With Sludge Wave, it is much easier to defeat than if it has Shadow Ball. Shadow Ball deals a significant amount of damage to Metagross.
Over time, Gigantamax Max Battles should become easier in Pokémon GO. One of the main issues is that most Trainers don't have high-level or effective counters due to Dynamax Pokémon only being available for a few weeks.
If you're a strong group of Trainers with a clear strategy of when to use Shields and healing, with your Max Moves fully upgraded and high-level Dynamax Pokémon, you might be able to take on Gigantamax Gengar with around 15 Trainers if it has Sludge Wave. We're still gathering more information about this, so check back soon for updates.

"Best Gigantamax Gengar Counters"
There aren't many Dynamax Pokémon to choose from in Pokémon GO at the moment. This limits the options. Gigantamax Gengar has four weaknesses in Pokémon GO, namely to Ground, Dark, Ghost, and Psychic-type attacks. We haven't had any Ground-type Dynamax Pokémon in the game yet. We also don't have a Dark-type, but Greedent can learn Bite.
As a result, the best counters for Gigantamax Blastoise are:
Dynamax Greedent
Dynamax Metagross
Dynamax Gengar.
These are the only few Dynamax Pokémon that can counter Gigantamax Gengar. By far the best choice is Metagross. This is a highly relevant Pokémon in the meta, so it's well worth investing in Metagross before you take on this battle.
"Pokémon GO Gigantamax Max Battle Tips"
You absolutely must be coordinated to win this Gigantamax Gengar Max Battle in Pokémon GO. Get on Campfire, see if you have a local Community Ambassador or any other nearby Trainers. Then, during the battle, you also need to stay coordinated. Have some Trainers use shields and healing each turn, and others attack. Ideally, you want one shield user, one healer, and two attackers per group of four.
Additionally, make sure you have good counters (if possible) and also reserve your best Pokémon for the end of the Raid. The Max Meter will charge faster at the end of the Battle. This is because once all your Pokémon are knocked out, you can still participate in the Raid by cheering. Cheering helps the Max Meter charge faster. So, even if you don't have the best counters, you can still contribute to your team's success.
If you have good Dynamax Pokémon in Power Spots, you might want to consider recalling them.
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