I just think I'll need help, just judging by how long it's taking me to grind for Fantina. And this is Diamond, in case you're wondering.
Make sure Torterra knows earthquake and Floatzel knows surf. Byron's strongest Pokemon is level 39, has 47 base special attack, and is 4 times weak to earthquake, so a level 36 earthquake Torterra and surf Floatzel should already be enough to defeat him.
If you still want to grind for some reason, then you should probably go to Iron Island. It has high-level wild Pokemon and some optional trainer battles. (it also gives you 10 physical defense EVs if you currently have less than 100 :P)
Also Riolu takes a lot of experience and is very likely not worth raising. Catch a level 28 Meditite on Route 211 (the east half) if you want a good fighting Pokemon. Those 28 levels come for FREE. Medicham has higher physical attack if you consider abilities, and it can learn high jump kick at level 32.