I'll give a set were Dialga can use whithout problem Roar Of Time ad OHKO most pokemon. Also with this set it will be easier to sweep evem with Dialga's lack of speed:
Dialga:@Adamant Orb
Nature: Modest
Trait:Pressure
EVs: 252SpAtk, 252Spd, 4HP
- Thunder wave
- Double team
- Roar Of Time
- Flash Cannon/Aura sphere/Ice beam
Strategy:Thunder wave helps Dialga to sweep because of his lack of speed when it also helps him dodge. Double team is here and helped with thunder wave makes Dialga one unhitable target.
Roar of Time can be used whithout problem since at the turn of rest it will evade the attack due to double team and thunder wave. Also it is the ultimate move that can OHKO any oppenent even with 2 X resistance. You can use Flash cannon for more Adamat orb abuse, Aura spehere for a multi-coverage and no-miss move, whe Ice beam is here for coverage.