Okay, so after about 98 hours and a year and some months, I finally beat the Elite Four and the League Champion in Pokemon Diamond.
This is kind of a big deal, guys.

Dialga - Level 55 - Equipped with the Adament Orb
Moves:
Iron Tail
Ancient Power
Dragon Pulse
Roar of Time

Rapidash - Level 56 - Equipped with the Razor Claw
Moves:
Fire Spin
Sunny Day
Fire Blast
Bounce

Luxray - Level 57 - Equipped with the Amulet Coin
Moves:
Shock Wave
Charge Beam
Crunch
Discharge

Empoleon - Level 57 - Equipped with the Splash Plate
Moves:
Bubble Beam
Waterfall
Avalanche
Hail

Roserade - Level 55 - Equipped with the Big Root
Moves:
Magical Leaf
Giga Drain
Leech Seed
Secret Power

Golem - Level 56 - Equipped with the Exp. Share
Moves:
Stone Edge
Rock Blast
Earthquake
Dig

I went in with about 30 Revives, 50 Hyper Potions, 15 Full Restores and 15 Max Potions. It probably took me about 10 times to finally beat it, and I would do the Elite Four often just to grind/collect XP and money but I usually had trouble with the Fire trainer. Soon, he got kind of easy but I never made it past the last Elite Four guy, the Psychic trainer. I went in with the intention of just grinding, but I used Dialga as my main for a change because he has so much HP, and the next thing I know I'm battling the league champion! I only had ONE PP restore, which was so stupid, but somehow I made it through slowly chipping away and using a lot of revives and potions! I was so psyched to have finished and I'm pretty excited to pick up Platinum soon.

The End