[Eomer of the Rohirrim]

I've been looking through some threads and consensus seems to be that 'we cannot know for sure'. Tolkien didn't give a detailed analysis of the Battle so we are left speculating. I guess you could argue for both ideas: Isildur either cut off the finger after Sauron had perished or as he was injured and fallen.

But to answer the original question, you could definitely stick a sword through him! Whether you were powerful or brave enough is another matter entirely.