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https://www.pokemonpets.com/Shiny-Mega-D...edex-10483

Shiny mega Dialga is one of the more versatile legendaries that is capable of fulfilling multiple roles on a team. It has an extremely good movepool, great typing and a crazy base 262 special attack! 
In fact, all of shiny mega dialga's base stats are at least 112, lowest being its speed.

It abilities are Expert Regenerator (Recovers 15% Hp at the end of the turn) Dragon Master (Boost dragons type moves base power by 40%) 
and Shield Master (The base power of opposing pokemon moves do 30% less damage) 

These are all beneficial abilities and enable Dialga to be used in several different roles. 
Role: Special attacker
Best Items: Epic hammer, dragon/steel orb/ special attack weapon
Best Nature(s): Modest, Mild, Rash, Quiet 
Recommended moveset: Flash cannon, Aura sphere, Roar of time/Dragon Pulse/draco meteor, Earth power. 

This moveset takes advantage of Dialga's ridiculously high special attack and ability of dragon master to inflict massive damage while restoring HP. Aura sphere has 1000 % accuracy, Flash cannon is steel stab, Earth power is an all around good special move , while the last slot can be filled by a dragon stab of choice.

Role: Boosting special sweeper
Best Items: Epic hammer, dragon/steel orb/ special attack weapon
Best Nature(s): Modest, Mild, Rash, Quiet
Recommended moveset: Flash cannon, Dragon pulse/Roar of time/ Draco Meteor, Protect/Magnet Rise, Round

This moveset boosts dialga's stats as evasion, special attack, special defense and defense. Round increases special attack one stage when its used, protect boosts evasion, defense and special defense, Magnet rise boosts speed which is extremely helpful because Dialga has a pretty low base 112 speed. The other two moves can be complimentary stab moves.

Role: Status Inducer and Trapper 
Best Items: dragon/steel orb, defense armor , special defense armor, 
Best Nature(s): Bold, Calm, Relaxed, Sassy   
Recommended moveset: Heal block, Dragon pulse, Thunder wave, Toxic 

This moveset is meant to inflict status on the opponent while prevent them from switching. It still has a dragon stab move with alot of pp in dragon pulse. 

Role: Elemental special attacker 
Best Items: Epic hammer, dragon/steel orb/ special attack weapon
Best Nature(s): Modest, Mild, Rash, Quiet
Recommended moveset: Flash cannon, Dragon pulse, Thunderbolt,
Icebeam.

Pretty much the same as special sweeper but instead a Earth Power and flash cannon, Bolt-Beam coverage is used to cover all the few resistances it already has.

There are many more roles that Dialga can perform effectively this are just the basic roles, Diagla can be used differently because of it's versatility.

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