It entirely depends on how much effort you are ready to put in. The foundation paper makes up the first half of the higher tier paper so the rest of the paper would get progressively harder. You should try out some higher tier past papers for your exam board and compare it to last year's grade boundaries and see where you lie grade-wise. In terms of improving your grade, try and do as many past papers and practice questions as possible. There are lots of websites online that provide practice questions as well as videos to explain tricky concepts. Good luck!