Board Thread:General Discussion/@comment-30406012-20180830141225/@comment-28362353-20180830142119

Zamasu couldn't do anything to Jiren though. Also, if trunks can destroy Zamasu, so can Jiren. We also know that Zamasu can be erased from existence (as seen by him being erased by Zeno), so Jiren could just fire a blast so powerful it erases Zamasu.

Also, Merged Zamasu is not Immortal. Since Black is mortal, that means Merged Zamasu is also mortal. Which is why he was destroyed by Trunks.