Is Mewnium Z a good held item for Mew?

Question

The move doesn’t seem too useful. I think something like a life orb or such.

Comments

Answer: No, Mewnium Z works better on Magikarp.

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This is for competitive, right? What format?

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sumwun, does it really matter?

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It depends whether Mewnium Z is banned. Also I think Mewnium Z is better in singles than doubles.

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Mewnium Z in singles. Not banned.

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It depends which kind of singles. I think it's banned in UU.

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In any singles format it’s not banned in.

Answer 1

Compared to other special attackers, Mew's decent bulk and ability to bluff will-O-wisp give more opportunities to set up, so offensive Mew is actually a thing in formats like Monotype (and also OU, but not as much). Against offensive teams, Mew can use rock polish to outspeed the fast and frail sweepers and one-shot them. Against defensive teams, Mew can use nasty plot to wallbreak. Then, when the opponent tries to use his/her/its best wall, genesis supernova can simultaneously nuke the wall, prevent revenge killing, and act as an additional setup move that powers up psychic. (note however, that opponents won't use walls if they have less than 30 points) If dark or steel Pokemon threaten to waste Mew's Z-move, then you can simply use aura sphere or focus blast for coverage.

Comments

What did you mean, opponents won’t use walls if they got less than 30 points?

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https://pokemondb.net/pokebase/meta/10194/

Answer 2

No

Mewnium Z makes Psychic into Genesis Supernova, a base 185 power Psychic type special Z-move that also generates a Psychic Terrain for 5 turns.

It's not worth using a slot for the following reasons:
- Mew is mostly used in a supporting role, so items such as Leftovers or Mental Herb is better.
- The Psychic terrain can also be set by other moves or abilities (Tapu Lele, if it's not banned) which makes this Z-slot and move redundant.
- It takes up a valuable Z-move slot that other Pokemon can make far better use of.
- The time it takes to switch Mew out for some other Pokemon to come in and take advantage isn't worth it, as these Pokemon usually only end up getting 3 turns of the terrain anyway

Given that most Taunt users tend to be Dark type, Mew usually needs to be switched out anyway rather than slapping on Psychic as its attacking move. The only use I see for this z move is in extremely niche circumstances for a gimmicky win at best.

Comments

Why can't Mew take advantage of its own terrain?

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Good answer, however sumwun gave more different uses.