The aim with everything is to memorise.
You just have to keep going over it again and again to memorise.
I did Edexcel GCSE Chinese and so I had to remember the characters (Hanzi) for writing and so it did take a lot of time for me to remember them (100-200) but I managed to do it and get an A in both of my writing controlled assessments.
Here's 2 ideas:
Code word each sentence, starting with the first letter of every word and memorise it like that. Try writing the letter for each word for each sentence on a flash card.
E.g. (I studied a bit of Spanish too, I remember the sentence - My birthday is on the 11th of July... Mi cumpleanos es el once de julio .. so on a flashcard I would write
M, C, E, E, O, D, J).
Flashcard each sentence and learn it one by one. I didn't try flashcards for Languages (writing exams) but these ideas that have come to my mind seem helpful.