Complex sentence: The Swiss mounted two attempts on Everest, pre and post-monsoon in 1952, led by Edouard Wyss-Dunant with Tenzing Norgay promoted to sirdar for the first time.
Split into simpler sentences: The Swiss mounted two attempts on Everest, pre and post-monsoon in 1952. The first was led by Edouard Wyss-Dunant with Tenzing Norgay promoted to sirdar for the first time.