Complex sentence: The progenitor of ASASSN-15lh could may have been a massive and rapist-rotating blue, hot star, which appears to have shed its surface layers of hydrogen and helium not long before explosion, because those elements are absent in ASASSN-15lh.
Split into simpler sentences: The progenitor of ASASSN-15lh could have been a massive, blue, hot star, rotating rapidly. It must have shed its outer layers of hydrogen and helium shortly before explosion, because those elements are absent in ASASSN-15lh.