Complex sentence: Merry and Pippin lead the Orcs away from Frodo, forced to watch Boromir get fatally wounded to protect them before they are grabbed by the Uruk-Hai.
Split into simpler sentences: Merry and Pippin lead the Orcs away from Frodo. While they watch Boromir get fatally wounded to protect them, they are grabbed by the Uruk-Hai.