Seriously, you can never really do well in English just by
learning the rules and going through vocabulary lists. I used to
shake my head sadly at my JC classmates who would spend hours
preparing for their GP by reading through lists of high-level
English words and their meanings. This in spite of the fact that
their basic grammar and vocab were already on shaky ground.

If you don't know how to walk, don't try to run, I say. There's no
point learning high-level bombastic words when your basic
foundation is really poor. All my GP essays were written in simple
English - I was more concerned with getting my points across than
to show off.
If you want to do well in English, then you've got to live and
breathe English. My tuition student got the shock of his life when
I told him that I have NEVER studied for any English test in my
entire life.
So he asked: 'So how did you do well for it?'
My reply? 'I read.'