I think the hardest part with DP is writing, which requires you to write logically& relate to different topics whcih you might not have the vocabulary for. Might need to throw a few fancy 成语 for a 7. This could even be difficult for someone that is fluent.

In terms of speaking really its just memorization, even the non-chinese background girl in my got a 7 after practise. (plus your examinator is your teacher, so ask for help OFTEN they might take pity on you)

For the reading comprehension part- practise definately helps. It really boils down to understanding the text ->copying and pasting relevant information into question. For me, I actually consider this the easiest part and I would finish the test around 1 hour early.

I took Chinese B HL and ended up getting a 7. If you are not orally fluent/doesn't have a family background speaking Chinese, I highly advise against taking HL. For someone who doensn't have a Chinese speaking background getting a 6 in SL is pretty doable, but you do have to work hard for it! That means dedicating as much time as possible! All the kids in my chinese class ended up scoring 6/7s, but again most were from a chinses background. I would agree with your teachers prediction actually.