Dialga the cover of Pokémon Diamond, is a very strong Pokémon, especially defensively. I believe it has only 2 weaknesses: Ground and Fighting. Whil its SP.Atk is the greatest, ATK is great too.
Item you shoud give is, of course Admant Orb, coz it increases both Dragon and Steel type moves.
Moveset (Special atk):
- Roar of Time / Dragon Pulse/ Draco Meteor
- Flash Cannon
- Aura Sphere / Flamethrower / Ice Beam
- Thunderbolt
Since Dialga has many options as attacks, i list the special attakc it should have.
- Roar of Time is its signature move, but it is like a Hyper Beam version Dragon, and the turn after can be dangerous. Dragon Pulse or Draco Meteor could be an alternative.
- Flash Canon is its strongest special steel type move, and it has STAB and boost with Adamont Orb.
The rest of the moves are options:
- Aura Sphere: Never misses and counters Steel type Pokemon who resist both STAB.
- Flamethrower: Also counter Sttell, and pairs well with Dragon type move, like with Reshiram, has aprently, almost no Pokémon resist that combination, like I learned in Reshiram POTW in 5th gen, but Ik now Fairy type exists, but Flash Canon handles them now, so...
- Ice Beam: Can be used to counter ground type Pokémon, who can have a super effective hit on Dialga with a ground type move.
- Thunderbolt: Pairs well with Ice Beam, has aparently it is a great combo. It counter's Azumarill too, since cox it is a Water Fairy, it resits both STAB (Immune to one of them even!) oh and Steel type hit neutrally, sorry.
It also seems that the only way for Dialga to counters fighing types is Aerial Ace, our Hidden Power Psychic or Flying, but i'm not sure if it worth it, coz Dragon Type moves hits them neutrally, and since they have STAB and Adamant orb boost, i believe they are stronger.
Counter for Dialga (I know so far):
Azumarill (If Dialga doesnt have Thunderbolt, and can strike with Super power), Fighting types in general. it is hard to counter...