Champions Barcelona will be bidding to get their Champions League title defence back on track Wednesday as Arsenal and AC Milan also look to keep the dream alive in the second round of elite European matches. But English giants Chelsea, who have already got their campaign off to a winning start, are set for possibly their toughest group assignment when they meet Spanish side Valencia at the Mestalla. Barcelona were held 2-2 by 2010 champions AC Milan at the Nou Camp in their Group H opener, but despite riding high after a 5-0 hammering of Atletico Madrid in La Liga in which Lionel Messi scored a hattrick, coach Pep Guardiola warned against complacency as the Catalans travel to Minsk to take on Belarus champions FC Bate Borisov. Both sides are level in Group H on one point after Bate drew 1-1 at Czech champions FC Viktoria Plzen. \"We are not unbeatable - any team can beat us. But if we do things right, the fans who come to watch us will have a good time, which is the most important thing,\" insisted Guardiola. \"We played very well (against Atletico), it was a lesson in concentration, but ... we have to do it again. Let\'s see if we’re able maintain this level. It\'s this year\'s challenge.\" Bate have lost their previous two ties against visiting Spanish sides Villarreal and Real Madrid, but Barcelona\'s David Villa, who has found the net five times this season, echoed his coach: \"Barca play a very high level of football, but each day is different and no team is unbeatable. \"But we sense that the fans are excited. They know how much potential we’ve got and what we can do, but we are aware that is only just starting.\" Chelsea boss Andre Villas-Boas\' side will be looking to take a big step towards the last-16 when they travel to Spain with striker Juan Mata just one month after he left Valencia for Stamford Bridge. Chelsea sit top of Group E after a 2-0 home win against Bayer Leverkusen with Valencia drawing at Belgium side Genk, who travel to the German club. Chelsea, who defeated Swansea 4-1 at the weekend, are unbeaten in their last six visits to Spain going back to 2005 and have won both previous trips to Valencia 2-1. But Villas-Boas knows they face a tough task at Valencia despite the Spaniards\' goalless draw at Genk and a La Liga loss at the weekend to Sevilla. \"The result at Genk was unexpected,\" said Villas-Boas. \"Genk is a difficult place to go and it\'s a very tight stadium with a good environment. But I was expecting Valencia to take the win. \"So I would say that this is a game where Valencia will try to get three points. \"A win at Valencia would put us in a very, very good position.\" Fellow Londoners Arsenal are second behind French club Marseille in Group F after a late goal gave German champions Borussia Dormund a share of the points. Arsene Wenger is keen his side claim maximum points from the three home fixtures against Olympiakos, Dortmund and Marseille to help them reach the 11-point target he has set to ensure qualification to the knock-out stages. The Gunners have a major doubt over whether winger Theo Walcott will be fit as they host Greek club Olympiakos after he suffered a knee injury in his side\'s 3-0 victory over Bolton, just their second of the Premier League this season.Marseille host Borussia Dortmund in the second Group F tie. In Group G, UEFA champions FC Porto travel to Zenit St Petersburg. FC Porto have won all four of their previous visits to Russia but Zenit were unbeaten at home in Europe last season. Porto won their opener against Shakthar Donetsk 2-1 as Zenit fell to Apoel Nicosia in Cyprus by the same scoreline. Apoel travel to FC Shakhtar Donetsk in the Ukraine.AC Milan host FC Viktoria Plzen in the second Group H tie. Milan, got their first win of the season in Serie A at the weekend, albeit against tailenders Cesena but have been boosted by the return from injury of Zlatan Ibrahimovic and captain Massimo Ambrosini.