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• Moves

Mr. Mime

Psychic is Mr. Mime's STAB attack and is well supported by his 100 base Special. His Speed is decent enough for him to use Thunder Wave. However, if you choose to use Thunder Wave, make sure to use it against a special attacker (such as Alakazam) because Mr. Mime has bad Defense. Thunderbolt is there so Mr. Mime can hurt Water Pokémon like Slowbro and Starmie, while Seismic Toss hurts Pokémon that it can't deal a lot of damage to, like Exeggutor.

Other Options

Using Reflect on Mr. Mime is an option since it helps his poor Defense. However, he has to be sure to set it up on a special attacker rather than a physical one. Mimic can be used too, and might pick up something to supplement Mr. Mime's limited movepool. If you play in an environment where you are allowed to play with Pokémon traded to and from GSC, then add Hypnosis over Seismic Toss. Mr. Mime can suffice as a cheap Parasleeper if that's the case.

Opinion

Mr. Mime is almost like a mini-Alakazam—although he has a better base Defense stat than Alakazam, his lower base HP stat counteracts it, and his lack of Recover greatly diminishes his usefulness. If you play in an UU environment, Mr. Mime is very useful as Poison-types are frequently used there. However, using him in OU is not recommended.

Counters

Exeggutor takes little from Mr. Mime's attacks, although it can't do much in return if Sleep Clause is already in effect. Chansey is more reliable in that she can fearlessly switch in and Thunder Wave Mr. Mime and Softboiled off any damage. Really, though, since Mr. Mime's defensive capabilities are poor, any physical-based Pokemon, regardless of metagame, that can survive a STAB Psychic and in turn hit Mime with a powerful STAB move serves as a reliable counter.