The best way would be to take a Pokémon with Sweet Scent in your party to either Route 119 or Route 104. Make sure Sceptile knows a spread move like Rock Slide. Use Sweet Scent to make a horde appear. Give Sceptile the appropriate power item, either the Power Anklet or Power Lens, available at the Battle Maison.
• Route 119 hordes contain Oddish, which yield Sp. Attack EVs. Have Sceptile hold the Power Lens.
• Route 104 hordes contain either Wingull, Zigzagoon, or Taillow, all of which yield Speed EVs. Have Sceptile hold the Power Anklet.
For more information, read here. Happy horde hunting!