Complex sentence: Kevin worked briefly as an assistant brand manager at Nabisco Brands selling cat food after earning the position from a summer internship as a young MBA student where his main task was to increase market share for Nabisco's biggest cat food brand.
Split into simpler sentences: Kevin worked briefly as an assistant brand manager for their cat food, after earning the position from his summer internship. His main task was to increase market share for Nabisco's biggest cat food brand.