Offering boost to 1.3 billion consumers in China, Beijing Olympics of 2008 attract huge sponsorship deals but marketers also find it a difficult battlefield, The Wall Street Journal reported.   While official sponsors are estimated to spend 2 billion US on advertising in the 12 months before the Olympics, Volkswagen invested 100 million US dollars in cash to win the official auto sponsorship.   Sir Martin Sorrell, chief executive of marketing heavyweight WPP Group PLC, reckons that by the time the Games come along, China could be the world's second-largest advertising market. Sir Martin has said it is "difficult to think of any sporting or cultural event in the world that could be bigger," quoted the paper   Advertisers face a raft of problems unique to China where sports marketing is still a nascent industry. Thirty-six companies have by now snapped up marketing rights to the 2008 Games, and the Beijing Organizing Committee is still choosing a final round of suppliers. "Few companies are succeeding right now in building unique associations," says R3's principal, Greg Paull. There is "tremendous potential for star associations to be 'overcooked,' " he says.