Well, I can help you with Explorers of Time (I played Darkness, but apart from a few game play changes, it's essentially the same).
Thing about Dialga is that you need to get lucky. When Dialga uses Roar of Time, it will need to recharge, meaning you essentially have a few turns to do something, though it'll probably be healing. If you're at full health, you should survive Roar of Time.
Ranged moves will be really helpful; if you can use a move from a few squares away, Dialga won't be able to stop you with its Intimidator IQ.
Additionally, the time I managed to beat Dialga, my Bulbasaur (aka, me in Pokemon form), had HP Ground. While that's not essential, having a Ground/ Fighting Move will really help.
Depending on where you are in the game now, I would suggest giving Piplup Brick Break. If you don't have the TM for it and are already in the dungeon, then make do with the best fighting/ ground moves that you can.
What you will need:
- plenty of Reviver seeds
- plenty of status seeds (especially sleep ones)
= I had a Bulbasaur which meant it could sleep power Dialga constantly; if you use seeds, you might find that you'll run out of them quickly
- plenty of Max Elixirs
It might take a few (or many) tries to beat Dialga, but just remember that you will need to go through the dungeon again, and inevitably level up as you do so, making your next attempts easier.