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While I'm here, I just realized that boosting won't be a problem. The reason is Transform!Scarf Mew. Not only is it bulkier than a lot of the things it's transforming into,
if you've trademarked a boosting move, you can't stop Transform(TM) by just not setting up. And on top of that, Mew will use the trademarked move
again upon transformation. Here's that mechanic in play with Download:
http://replay.pokemonshowdown.com/customgame-391744661
So while Swords Dance(TM) might come off at first as a slightly worse version of Huge Power on Garchomp, in practice you better have a solid switch-in to scarfed +4 Garchomp if you don't want to get 5-0'd. In practice, offense teams that want to abuse trademarked setup moves will need to engage in the kind of imposterproofing you see in Balanced Hackmons. And if Mew gets its Choice Scarf knocked off, it will probably be quite valuable against defensive teams.
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Substitute, though it cuts into your HP automatically, might give you that free turn you need to set up
and imposterproof you all at once. But it backfires if you lose your Substitute in the process of KOing something, then Mew comes in, boosts up to you, and then it sets up a Substitute of its own... terrifying.
And now we have Imposter Mew... Great...
So similar to Imposter Chansey with Eviolite...
Expect Mew can be even faster and stronger than oponent IF didn't hold Scarf, where Chansey is just more bulky than oponent.
The trick is creating entire team with counters to other members of own team. Similar to Chansey with Eviolite, Mew is nearly always better version of copied oponent, BUT have only 5 pps for all 4 moves.
First thing is creating sweaper, who isn't weak to itself. The only one pokemon, who truly is Iposterproof is... Ghost Arceus with Jugment and Fighting type coverage move. But because Arceus is in Ubers, Mew is safe for sure... Is that true?
Not that quite like you expect. Still we have Hidden Power, what can be really stromg move against Mew if is not prepared.
Heatran sometimes carry HP Ice for that persky Ground types, but can also carry HP Ground over Earth Power just for being not weak to copy of itself. Mew will propably have Dark HP, but will propably switch against Heatran after checking moveset. The only one thing what we gain in this proces is one free turn, but if oponent have Counter against Heatran, we anyway can't do anything...
Another idea could be pokemon, who is immune to all moves what copied Sweaper has.
For example: Garchomp is Hardly counterable by Fairy with immunity to Ground IF have only Dual STAB set + Setup moves... just put Ballon on Fairy pokemon or use Magnet Rise(TM)...
Another Example: Calm Mind, Rest Talk Suicune is counterable by pokemon with Immunity to Water IF Suicunes only atacking move is Water Type.
Another One: Flash Fire Heatran with Ballon completly wall Volcanion with Earth Power, Sludge Bomb and Fire STAB move. Wihout Signature move... Normally Volcanion always take Steam Eruption, because is just amazing water move, but oponent didn't know about it... until will use Mew against Volcanion...
How effective could be this team against other, real threats?
...and here we have problem... Even when we build perfectly counters against all team members, still we also must consider oponents other threats...
Interesting coutner against Scarf Mew could be Unburden pokemon with activated ability. Mew copy oponents ability, but didn't activate it, expect Mew will lose item. Only Counter - Trick Room. If oponent have Trick Room, can even send Scarf Mew against Unburden pokemon and being slower atack first under Trick Room. If we switch, we lose boost for speed... and then we lose later (when Trick Room ends) to Scarf Mew...
I wonder... if Mew is too strong with Transform... it will be even legal in this metagame?
Yeah... I feel incoming ban for Mew with Transform... I have just hope I'm wrong...
Someone could maybe find true counter to Transform Mew?