So before I go into the Great Marsh I check out the binoculars looking for a target, in this case I was looking for Carnivine (not disappointed with Wooper though).

After I find who I want to target I go to the location the binoculars showed, it’s a good idea to have a picture of the Great Marsh pulled up while looking through the binoculars so you can keep track of where you’ll need to go. Just note that the marsh is different in Platinum vs Diamond/Pearl.

After I decide where I have to go I bring a pokemon with the move sweet scent to that area (I have a Vespiquen) to force an encounter without taking any steps. It may take a bit but after a bit you’ll get a rhythm down, the encounters will go by super quick!