Hi. What would be a good catch phrase or slogan for the Mountain(radical group of Jacobin) against Louis XVI during his trial

Answer 1

Judging by what I was able to learn at this website, in the part dealing with the sans-culottes,
http://www.wsu.edu/~dee/REV/RADICAL.HTM ,

I would say that "Off with his head" (or their heads) would be an appropriate slogan expressing their views.