Get yours a couple years worth of past exam papers and work through them. Read up yourself or ask your teacher to explain the bits you don't understand. [Especially the completion of the melody my teacher helped me alot with that.]
Grade 5 really isn't that complicated, Intervals are tricky but after a while they just click and you realise the simplicity in them [I'm quite mathmatically minded so this might be harder for you]
In the same way interpreting chords from a score becomes quite simple after a while [especially the cadences at the end Hint: They're nearly almost always II V I]
I never really learnt the grouping of notes and that came up so I'd learn that

. Also vocabulary is important but again only makrs up about 2 marks in the whole paper.
If you know your key sequences [Although it's really easy to work them out simply by running through the circle of fifths in your head] the scales aren't hard to write out at all.
Transposing is incredibly tedious, but I wouldn't call it hard. It's quite easy to make careless errors though.
You could easily do this is you comitted yourself fully, good luck.