All hope is not lost!
Dialga gets Bulk Up, which is an awesome boosting move that helps to take advantage of Dialga's stellar typing and resistances to snag a few boosts. In Gen V the Bulkup Resttalk set was effective, and it used a Careful Nature Dialga. Now that there are Fairy types you can no longer run mono Dragon as an attack, but you can run a more offensive Bulk Up variant.
It carries a surprise factor too.
Dialga @ Chesto Berry
Ability: Pressure
EVs: 252 HP/252 Atk/4 SpD
Adamant Nature
-Bulk Up
-Rest
-Iron Head
-Dragon Claw
Or you can ditch Rest altogether and run Bulk Up + 3 Attacks.
Dialga @ Leftovers
Ability: Pressure
EVs: 252 HP/252 Atk/4 SpD
Adamant Nature
-Dragon Claw
-Iron Head
-Earthquake
-Bulk Up
Hope I helped!