It was a huge feedback loop, but it looks like I was a little wrong. I looked it up and it turns out that you can go back to Treasure Town (where your home base is) to do other dungeons and stock up on items again, but then you need to clear all the dungeons on the way to Dialga again. I didn’t know that back then, though, so I just kept trying until I got lucky and found the right items in Temporal Spire, the dungeon that you have to go through before fighting Primal Dialga (because a checkpoint right before the final boss would be too easy). The design philosophy probably was “This is the final boss of the main story, make it difficult.” Dialga has Roar of Time, for example. That move can hit you anywhere on the floor and deals a ton of damage.
It was a huge feedback loop, but it looks like I was a little wrong. I looked it up and it turns out that you can go back to Treasure Town (where your home base is) to do other dungeons and stock up on items again, but then you need to clear all the dungeons on the way to Dialga again. I didn’t know that back then, though, so I just kept trying until I got lucky and found the right items in Temporal Spire, the dungeon that you have to go through before fighting Primal Dialga (because a checkpoint right before the final boss would be too easy). The design philosophy probably was “This is the final boss of the main story, make it difficult.” Dialga has Roar of Time, for example. That move can hit you anywhere on the floor and deals a ton of damage.