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Mega Gengar was my go to mega in Battlespot last gen, the dude was so good and could win games for you by himself. Enter Gen 7, and regular Gengar loses Levitate, which along with the nerf to Mega Milfs Parental Bond and the nerfs to the -ate abilities was a sign that GameFreak were beginning to reign in the major threats of the previous Battlespot seasons.
Losing Levitate hurt Gengar since it meant he could no longer switch in on threats such as Chomp, Lando-T, Excadrill and Mamoswine, which was a major blow, since Gengar would previously have been able to force out Scarfed Lando, Excadrill and Chomp users and gain momentum for your team.
This was a setback and would have been ok in isolation, but then Destiny Bond got BTFO'd as well, with GF changing it's mechanics to work similar to moves such as Protect and thus no longer spammable.
The final nail in the coffin however was the introduction of a certain Tapu named Lele and her terrain. Initially Lele wasn't thought of as a threat since M-Gengar naturally outsped her, but eventually people figured out that due to the combination of her high SpA and her ability, Lele didn't need to use Specs or Life orb, she could just use Choice Scarf to outpseed the main threats to her, such as a certain M-Gengar, and BTFO them, whilst still retaining the ability to do a fuckton of neutral damage to others.
Once the Scarf Lele set became the most common set. as well as Lele herself becoming the single most used mon in Battlespot singles, Gengar was done for. He could possibly have survived the first two punches and been able to retain his spot as one of the best mons in the tier, but the third punch from Lele was a knockout blow, and now Gengar has fallen from grace.