Giga Drain isn't good for Sceptile. Leaf Blade is the way to go.
Timid is the best nature, but out of those choices, go with Modest.
Sceptile (Timid Nature) @Leftovers
EVs: 252 Speed, 214 Sp.Attack, 44 Hp
-Leaf Blade
-Substitute/Safeguard
-Leech Seed
-Thunder Punch/Dragon Claw
Sub is wayyyyy better than Safeguard. It's just another option. :/