Lucario is one of the worse Pokemon in Platinum, mostly because it hatches from an egg. If you have the time and patience to level grind a Lucario, that time and patience is better spent on leveling up your other Pokemon. The best fighting Pokemon in Platinum (besides Infernape) is probably Gallade.
Gallade @ fist plate/mind plate/expert belt
EVs: 100 Spe
- zen headbutt
- close combat
- swords dance
- night slash/X-scissor
Another option is to simply put your Garchomp in a PC box and not replace it with anything, so the remaining Pokemon on your team can get more experience and reach higher levels. You don't need a fighting Pokemon on your team.