I went to high school in Shandong, and none of our school was admitted to No.1 Middle School.
In terms of difficulty, it's easy. But the problem is that our students are going to Beijing for exams, and the whole class can be admitted to a university. So if you are a student in Beijing, it is almost as difficult for you to take the Tsinghua exam as it is for you to take the Japanese Dongda exam. However, considering that the unified examination for international students in Japan is less difficult than the national college entrance examination. So if it's easier than the exam, it's still easier.
Ps Dongda University has high requirements for students' comprehensive quality, and good universities in Japan have to interview, which is almost the most important and difficult part. If the interview is not good, no matter how high the score. You can see the comprehensive quality of students during the interview.
Want to go to Dongda depends on strength and Japanese grades. Performance in the interview (wider knowledge).