Sluggish sales at its largest businesses. A need to grow faster than the industry and gain more market share. Pressure to move into more rapidly growing niches. Boston Scientific Corp.’s CEO since November, Mike Mahoney, outlined the challenges on Tuesday. He forecast the company’s core markets will stabilize this year and grow 2 percent in 2015 and 4 percent in 2017. The stock had been rising since December but fell 0.6 percent on Tuesday and 1.8 percent on Wednesday.