Who was the most powerful elf at the time of LOTR?
That depends on your definition of powerful: With her ring Nenya, Galadriel was probably the most powerful defensively; however, if you mean offensively, it was probably the Elf Lord, Glorfindal.
Also was Beleriand a different land mass than where LOTR took place?
Before it sank beneath the waves at the end of the First Age, Beleriand was part of the same land mass. It was located to the west of Eriador on the other side of the Blue Mountains and Forlindon; that is, it was far to the west of The Shire.