Shadow Dialga

Dialga has always been a strong but outclassed option. But the Shadow boost makes it substantially better. Despite the TDO loss, its Steel sub-typing keeps it very strong on the bulk side when facing down other dragons. On paper it looks a bit underwhelming, but this thing manages to mix relatively high DPS with surprisingly high TDO against Dragon Type movesets, making it a very strong Pokemon that can definitely help out any Dragon Type team.

Best PvE Offensive Moveset

• Dragon Breath should be paired with Draco Meteor for top TDO against Dragon-types.
• Metal Claw should be paired with Iron Head for top Steel-type DPS.
• Thunder isn't useful on Shadow Dialga.

Dragon Breath is, frankly, the main source of Dialga's offensive power, and is thus the correct choice for fast move. It provides an incredible amount of consistent Dragon-type damage.

Iron Head is Dialga's cheapest move, and while it is (on its own) mediocre, it provides solid anti-Fairy coverage, and is generally good enough.

The only real decision you have to make in your Dialga's moveset is between Thunder and Draco Meteor, and you'll find players who swear by each. Draco Meteor is a one-shot nuke, which can deal a huge amount of damage to everything in the format, but at the cost of dropping your attack to unusable levels after. Thunder gives you more power against Flying-types and Kyogre, but doesn't hit quite so hard against the rest of the format. Experiment, and see which you like more!