Instructions
Travel to either Route 42 or Mt. Mortar -- Marill can be found in both locations, which are east of Ecruteak City.
Walk in the tall grass or inside of Mt. Mortar until Marill appears.
Attack Marill until its health bar, located underneath its name, is red. Do not use electric or grass attacks, as Marill is weak to those and you might accidentally defeat it.
Use any status changing moves, such as sleep or paralyze, to make Marill easier to catch. Avoid moves like poison or burn, as those do damage over time and may accidentally defeat Marill.
Use the strongest Pokeball you have to capture Marill once it has been weakened.