Dont really know what to tell you here since ive never had problems in English. I guess improving your vocabulary is very important. One quick way to do this, as ive learnt recently, is to read classics. They use some heavy language and keep you entertained at the same time. Also, a lot of difficult words are repeated so they kinda stick with you for a long time. Reading will help you a lot, both in your paper and pretty much everything else imo.
In the writing section of the paper, i would always pick descriptive writing. Its the easiest. Just describe everything you see, hear, touch, taste and smell with as much detail as you possibly can.