The B2 First and C1 Advanced exams from Cambridge Exam English are both similar with regards to their format. For part 2 of the exam you need to fill in the gaps in a text with just one word, Click HERE for an example. The gaps are all based on grammar structures, so you must therefore look at the different grammatical forms needed to pass these exams, for example HERE.
The best way to improve this part of the exam is to improve your understanding of English grammar. You should study this list and read English texts frequently.
Study this list of frequent answers to make part 2 of the FCE (B2) and CAE (C1) exam easier:
Articles: a, an, the
Determiners: so, such, not, no, this, that, these, those, if, unless, all, every, each
Auxiliaries: should, may, can, could, might, will, would, (be, have, do = all conjugations), must, ought to,
Prepositions: on, at, in, out, up, down, for, with, into, around, about, like, through, over, under, before, after,
Adverbs: since, yet, still, already, just, for
Relative pronouns: which, who, that, whose, when, where, why, whom, when
Quantifiers: much, many, (a) little, (a) few, a lot (of), some, any, no, plenty (of), none, both, all, either, neither, each, every, (the) other(s), another, little.
Connectors: however, but, whereas, although, even though, though, while, and, in addition, besides, also, too, for example, for instance, like, such as, because, as, since, because of, due to, on account of, owing to, In conclusion, to sum up, in brief, therefore, to summarise, on balance, in summary, so, therefore, thus, hence, nevertheless, moreover, when, firstly, secondly, lastly