Here's what I think Ash needs to do and it is exactly what Paul has done. In DP 131, Reggie asks Brock if Ash has any strategies for the match like substituting his pokemon for the match against Paul. Of course, Brock/Dawn reply that Ash is using the same six pokemon. Well Reggie goes on to explain that Paul has intentionally changed his team around specifically cause of the previous battles with Ash and that is why Paul can predict what Ash does in the battle. For example: Paul didn't swap out Torterra even though Staraptor came out. Why? Paul had Stone Edge waiting for Ash's Staraptor. Or in DP 132, when Pikachu Volt Tackled towards Magmortar, Paul had Magmortar use that move on the ground to slow him down.
In the team Paul used against Ash, his team has 3 members 2x weak to rock. So maybe Ash should start teaching his pokemon rock moves or teach his pokemon moves to counter Paul's counter to his pokemon. Grotle should learn Stealth Rock for Honchkrow. Buizel should learn ice punch, ice beam, or blizzard to counter Torterra. As mentioned by others, Pikachu should learn feint to counter Electabuzz or Surf for Magmortar(yeah right). Anyways, those are just some examples, but I don't think Ash should totally change his style. He just needs to be better prepared against Paul. Paul did his homework on Ash, Ash needs to do the same in order to compete against Paul.
PS: Sorry for the wall of text.
In the team Paul used against Ash, his team has 3 members 2x weak to rock. So maybe Ash should start teaching his pokemon rock moves or teach his pokemon moves to counter Paul's counter to his pokemon. Grotle should learn Stealth Rock for Honchkrow. Buizel should learn ice punch, ice beam, or blizzard to counter Torterra. As mentioned by others, Pikachu should learn feint to counter Electabuzz or Surf for Magmortar(yeah right). Anyways, those are just some examples, but I don't think Ash should totally change his style. He just needs to be better prepared against Paul. Paul did his homework on Ash, Ash needs to do the same in order to compete against Paul.
PS: Sorry for the wall of text.