Spawning Mechanics
The Great Marsh is divided into six zones. Every day, one of ten special Pokemon spawns in each zone. "Pokemon: Diamond" uses the Nintendo DS system clock for the time, so this is a real world day. Skorupi can only appear in the Great Marsh before you get the National Pokedex. Drapion replaces him once you get this during the main quest. There are no additional factors affecting whether or not Skorupi appears. You just need to check once a day until he is present in one of the areas.
Checking Special Pokemon
You do not have to enter the Great Marsh and explore each area to find out if Skorupi has spawned. You can check what special Pokemon are available and which area each one is from the welcome center. On the second floor is a pair of binoculars. When used, you are shown each of the six areas of the Great Marsh. The special Pokemon in that section is wandering around when viewed. Skorupi appears to be a purple scorpion. It costs $100 to use these binoculars.
Saving
You cannot save your game while in the Great Marsh, which means you cannot save immediately before attempting to capture Skorupi. You can save right before you enter the Great Marsh. This does not have any real significant advantage other than allowing you to avoid paying the $500 entrance fee for the Great Marsh more than once. Walk directly to where you saw Skorupi with the binoculars after entering the Great Marsh. Run from any Pokemon you encounter on your route. It should take less than a minute to reach Skorupi from your save point.
Capturing Skorupi
You have three options when attempting to capture Skorupi. You can throw mud to make him easier to catch. You can throw bait to make him less likely to run away. In order to capture, Skorupi you must throw safari balls at him. The easiest way to capture him is to throw a safari ball every round and ignore the mud and bait options completely. Each safari ball has about a 25 percent chance of capturing Skorupi. It should only take about four or five rounds to capture him.