Barcelona assistant coach Jordi Roura says his team always finds it difficult to defend against Real Madrid star Cristiano Ronaldo.The Blaugrana travel to the Santiago Bernabeu on Wednesday for the first leg of their Copa del Rey semi-final against the Blancos.Roura has warned his players there is no easy solution to stopping Jose Mourinho's men, and highlighted the danger of their No. 7 in particular."We know about Cristiano's qualities, but it's always difficult to stop him," the coach told reporters on Tuesday. "We have to be concentrated. There is no magic wand."Cristiano is a great, great player. It's understandable he is scoring goals against us now when before he wasn't."Barcelona are 15 points ahead of third-placed Madrid in La Liga, but Tito Vilanova's assistant insisted that the Copa del Rey is a completely different proposition."I understand people commenting on how well we are playing," Roura added. "But they will be very dangerous. We are playing in Madrid against Real Madrid and that's always difficult."In the league we are in a comfortable situation. We have earned that. This is a different competition and we have to try and achieve a positive result there [in Madrid]."With the squad they have, I don't think Madrid will be conditioned by the players they have missing. I expect an intense and complicated Madrid. I don't think it will be any different than it normally is. They are super-competitive."Arsenal continue to be linked with out-of-favour Camp Nou striker David Villa, but Roura denied any knowledge of the Spain international being set to be sold any time soon.He added: "I have no proof of any Arsenal interest in Villa. He is our player and that will stay the same until the end of the season."Barca defeated Malaga 6-4 over two legs in the quarter-final to set up this latest Clasico clash, while Real Madrid went through 3-1 on aggregate against Valencia.