How do you beat the 4th gym leader on Pokemon platinum?

Answer 1

Since you named the specific game I can answer, as you probably know, the leader, maylene, uses a meditite, a machoke and a lucario, the type coverage sheprovides is quite scary as there is no lone Pokemon that is a counter to all 3, I recommend bringing a ghost type Pokemon seeing as that is immune to all fighting type attacks, maylene's strongest moves, its a perfect counter to meditite, as your ghost type attacks provide super effective coverage, you will have to resort to something else when fighting lucario though, and if machoke has foresight you could be in a lot of trouble.

In any case that Machoke does have foresight, bring a strong psychic Pokemon (the hardest type to get a decent Pokemon from easily) and the easier to get flying type, the best resort is to use a bug flying combo, the best example being vespiquen, (again, very difficult to get) but a staravia will do.

As for Lucario, your best bet is to use your starter Pokemon, but if you chose turtwig and it isn't a torterra yet, (it evolves into that at level 32) the best solution would probably to get a hippopotas (leave veilstone city using the south exit, go to the ruin maniac's cave, and depending on certain conditions, a 5-20% chance of it appearing) and train it to about lucario's level, if you can't really be bothered to get a Pokemon that rare, then another option is gyarados (but you might find it very difficult to evolve magikarp), other alternatives are machoke, buizel and floatzel, shellos and gastrodon, ponyta and to a certain extent, bronzor (maybe even a bronzong by then).

Overall, meditite and machoke shouldn't be a problem, but lucario will be difficult to beat, even with the list of counters already mentioned, but if you chose a starter Pokemon, (particularly chimchar) it should be fine.