I've been accepted to Cardiff Sixth form, and thought I should post as I had these same questions myself. The entrance exam is not hard, if you learn the majority of the content of your chosen subject to do the exam in. It is basically a 1hr GCSE paper. There is an informal interview, where they ask you why you want to go to the school, and your hobbies etc. There is also an aptitude test, involving maths and non-verbal which is quite hard, especially towards the end.
Good luck!
It is a very academic school, but there are amazing opportunities especially for medical students. Please make sure it is the right environment for you though, as it's not the best for everyone! Be prepared to study hard, and encounter challenges, but remember there is fun as well. There are many school trips, and extra curricular clubs, and even a free membership to the amazing David Lloyd clubs!
All the best x