Best Answer: Well for starters you could try to diversify its moveset. Never mind that Serperior has a shallow movepool, there is no reason to give a grass pokemon three different grass-type moves. If you change up its attacks, it will be much stronger.
You also didn't tell us what nature it is, which can play a big role when choosing attacks. If you're going to ask for team help, be sure to always post natures and abilities.
You will need two Heart Scales to fix Serperior. You can find them laying on the beach of Undella Town & Undella Bay if you turn on your Dowsing Machine. You can also get a free one each day by talking to a girl in Driftveil and showing her a certain move.
Once you get the Heart Scales, go to Mistralton and speak with the Move Relearner. She will reteach it Coil and Leech Seed. If you haven't already gotten the TM for Aerial Ace, you can get it in Mistralton as well. Speak with the kids in the house by the Airport and then go check the runway to recover the TM.