Things You'll Need
Instructions
Insert your copy of "Pokemon Diamond" into your Nintendo DS or Nintendo DS Lite, then put "Pokemon Sapphire" into the Game Boy Advance (GBA) slot.
Turn on your Nintendo DS, launch "Pokemon Diamond" and select "Continue" from the main menu.
Buy plenty of Pokeballs at the nearest Poke Mart. Dusk Balls have the highest capture rate of any Pokeball other than the Master Ball as long as they are used at night, but if you are unable to play later in the day, purchase Ultra Balls instead.
Prepare your team for catching wild Pokemon. You will want at least one Pokemon with a sleep-inducing move like Hypnosis, Sing, Yawn or Sleep Powder. Another Pokemon should have the move False Swipe. Put the False Swipe user in the first space in your party. Wild Pokemon in the area you'll be visiting range in Level, from 16 to 18, so bring a team that's significantly higher leveled to avoid taking too much damage.
Walk or fly to Hearthome City, then travel westward to reach Route 208.
Walk around in tall grass until you encounter a wild Seviper. This may take a while, as other Pokemon are present in the wild on Route 208. Keep trying until you find one.
Attack Seviper with False Swipe, which should reduce it to 1 HP without knocking it out.
Switch Pokemon, sending out the team member who knows a move to induce sleep. Use the sleep-inducing attack to cause the Sleep status effect, which will make it easier to catch Seviper.
Throw the Pokeball of your choice until you succeed in capturing the wild Seviper. You may need to put the Seviper back to sleep if it wakes up before you capture it.