On Mew: Honestly, I really like Mew. Full SpD makes it a nice blanket check vs. special threats particularly mega diancie. PhysDef makes it a nice blanket check vs. physical attackers. One of the few pokemons that can counter medicham long-term. Unfortuantely, it has a severe 4MSS. Usually when I slap it on a team it's acting as a compression role glue with either defog or rocks. This time I wanted to build an entire team dedicated to the stallbreaker set, as it is super annoying to deal with for any team that isn't HO. PhysDef for medicham + speedy to taunt tank chomp and outspeed bisharp and a good amount of bulky excas. Knock off for crippling its usual switchins such as rotom-w/tornadus-t/heatran/clefable.
On Manectric: Since Mew is a solid and fast taunt user, I wanted to build spikes. I've also been wanting to build around Mega Manectric recently. It takes advantage vs. common balance builds that rely on exca/lando-t/chomp as their elec immunity. Unless max/max excadrill, it's a very high roll to 2HKO w/ flamethrower. But with spikes putting a lot of pressure + rocky helmet ferro + Jellicent in the back excadrill is going to be overwhelmed quite easily actually. And then the teams crumble. Also Manectric is insanely good vs. HO. Outspeeds serprior and checks manaphy, scizor, and thundurus.
Team: Jellicent is SpD for mega diancie but enough defence for diamond storm and bulky exca. Toxic + Scald so that it can actually 1v1 mega diancie and volcanion instead of just sitting there. Jellicent also serves as a second taunt user to maximize spikes pressure. Ferro is t-wave for clef/heatran/zard/volcarona switchins. Torn-t is superpower to whiddle heatran and smack down to serve as a volcarona check alongside jellicent.
Threats: Weavile is probably the most noteworthy threat. Keep ferro healthy for weavile, rocks up, and evolve manectric as early as possible. Mega Zam can be played around with but it's always a looming threat.
Tips: Calc lopunny damage. If adamant manectric outspeeds which is huge.