SEOUL — The slump in Samsung Electronics that wiped out $28 billion of market value in six weeks will deepen as Apple and Chinese rivals take market share in handsets, according to the stock's most-accurate forecaster. Shares of Suwon, South Korea-based Samsung fell 13 percent since Nov. 29, losing more market capitalization than any other company worldwide. The stock will sink another 11 percent, said Adnaan Ahmad, an analyst at Berenberg in London whose recommendations during the past 12 months produced the best return among forecasters tracked by Bloomberg. Samsung, the world's biggest smartphone maker, posted its first profit decline in nine quarters in the final three months of 2013 amid growing competition from Apple's iPhones and budget devices from Chinese producers. "Selling is totally justified because the market now understands that the margin profile will change drastically," Ahmad said Jan. 7. "Samsung is in a very precarious position in the next 12 to 18 months."