The simple answer is depends on the time you have to prepare, if you have 4 years to prepare then you have enough time to get a good grade!
In recent years students in Hong Kong who go to local schools have DSE Spanish as a new elective subject to choose from. However in most local schools they do not teach Spanish as a foreign language. This means some students consider taking Spanish as an elective subject with the help of a tutoring center so that they can drop another subject. These students however underestimate the difficulty of the Spanish DSE exam.
If you choose Spanish, this means you are expected to learn it though out your WHOLE secondary education, that is 6 years! So if you expect to be able to prepare for DSE Spanish in 2 years that is totally impossible. With the help of a good tutor, it would be possible to prepare for this exam in 4 years or if you are very smart and disciplined even in 3 years, but less than that for sure you will fail.
In fact the HKDSE Spanish exam is a copy of the AS A levels Spanish exam from UK. Since the Hong Kong Education Bureau cannot make their own exams for foreign languages, they use the exams from Cambridge AS A levels to examine the students.
So how long a student prepares for the AS A levels Spanish exam in the UK? The answer is 5 years, 4 of compulsory education 11-15 years old + 1 more year when students are around 16 years old. Therefore expecting to pass this exam by studying Spanish only for 2 years would be totally unreasonable!
As a summary, HKDSE Spanish is not very difficult but you will need enough time to prepare and learn the contents so be aware of the investment in time that you will need to make!