Question

Is Mega Diancie with Magic Bounce a good Pokemon for setting up Light Screen and Reflect?

Comments

Not a full answer but not really, it wants Light Clay to extend screens turns and being a mega evolution causes the mega stone to take its item slot. If you want a Fairy type Pokemon that sets up screens your best bet would be either Grimmsnarl, Tapu Koko, or Klefki, whichever is available. If you want a rock type you can try Glimmora or just base Diancie with Light Clay.

The reason why this isn't a full answer is because I can't back it up with proof lmao

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I use Grimsnarl it works great, idk  about Diancie

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What format are you trying to use Light Screen and Reflect Mega Diancie in?

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Please specify the format and generation you are planning to use Mega Diancie in. Generally, I dont think its a good idea to use Mega Diancie as a screens setter because it cant hold Light Clay and takes up your Mega slot.

Answer 1

No, Mega Diancie is not a good Light Screen and Reflect setter for the following reasons.
1. Mega Diancie cannot hold the item Light Clay because its item slot is taken up by the Mega Stone. Light Clay extends the duration of Light Screen and Reflect from five to eight turns and is thus almost mandatory for a good screens setter in competitive play.
2. Mega Diancie takes up your mega slot and thus prevents you from using a different mega like Tyranitar, Metagross, Scizor, Gyarados etc.
3. It doesnt have access to a switching move like U-Turn to take itself off the field after setting up its Screens and thus allowing strong attacker to come in.
4. In Gen 6 OU, it is outclassed a Screens setter by Serperior and Latias. In Gen 7 OU, it is outclassed as a screens setter by Tapu Koko and Alolan Ninetales.
Note: The above answer is for 6v6 Singles in gen 6 or gen 7, especially Gen 6 OU and Gen 7 OU.
Source: experience in gen 6 and gen 7 competitive singles

Answer 2

Nope. Use trick room Hatterene or, alternatively, Espeon. Save your mega for an offensive mon.