Pokemon GO trainers should definitely plan to get a Mega Gengar, since this Pokemon is simply too powerful to pass up on.
Gengar was one of several Generation I Pokemon to receive Mega evolutions in Generation VI (Venusaur, Blastoise, Gyarados, etc.). In both the main series games and in Pokemon GO, Mega Gengar gets enormous offensive stats that allows it to steamroll many opponents. Any trainer who picks up this Pokemon is guaranteed to perform better in PvP and Raids.
Mega evolution becomes Mega Tier Raid Pokemon
Until October 22, 2021, Mega Gengar will be the Raid boss for the Mega tier. This should give trainers several weeks to try and beat Mega Gengar in a Raid battle and potentially catch it.
Without a doubt, any trainer with three allies or less is going to fail at this Raid battle. Mega Gengar is so strong that it can melt Pokemon’s health away in seconds.
Since it’s such a huge threat, it is highly recommended that trainers reach out to the community for help with this Pokemon. Whether it be through Discord, Facebook, or another website, trainers who have multiple friends in the area should team up as much as they can to face Mega Gengar.
Just to prove how powerful Mega Gengar is, Deoxys Defense Forme is a good Pokemon to test Mega Gengar's strength against. It's a Psychic-type with one of the highest Defense stats in the game (330). Despite the fact that it's so bulky and does super-effective damage in return, Deoxys Defense Forme as a Raid boss still drops to Mega Gengar in 765.9 seconds.
Mega Gengar can even destroy Pokemon that it doesn’t have a type advantage against. Looking at Lucario’s Raid counters, Mega Gengar is the 26th ranked Pokemon, behind several Fire-types.
Of course, since this Pokemon is clearly a threat to be reckoned with, trainers are going to need Pokemon who counter it. In this effort, Dark-type are going to be a trainer’s best friend. These Pokemon resist all of Mega Gengar’s attacks while applying huge damage to it.
Examples of highly ranked Dark-type counters to Mega Gengar would be Tyranitar, Weavile, and Mega Gyarados. Since Mega Gengar is weak to Psychic, Mewtwo is also a counter, but it’s risky since it easily dies to Mega Gengar as well. Also, oddly enough, Mega Gengar counters Mega Gengar.
Of course, even after beating Mega Gengar, trainers will still have to catch the Pokemon. This is definitely a difficult task, but any trainer who fails to catch it doesn't need to worry. Gastly will have a Spotlight Hour on October 12, and trainers can use the Mega Energy from the Raid fight to evolve Gastly into a Mega Gengar.