You could try searching for wild Chanseys in an attempt to obtain a Lucky Egg on Route 209, Route 210, and the Trophy garden, but it's a rare encounter, and Chansey only holds the item 5% of the time, so it will probably be very time consuming unless you're a really lucky person.
If you can't be bothered to search for a Chansey, I'd recommend just going to a Route where your Lucario can KO Pokemon easily and train there for a while, then move onto a route with higher level Pokemon to battle once you've trained a bit. Basically just keep moving through routes until Lucario is a high enough level to have a chance at beating the Elite Four. It takes a long time, but it's how I've always done things.
Hope I helped!