If you are just finishing high school and have passed your science exams then the assessment shouldn't be a problem for you - assuming you did well and have "fresh knowledge". It's really hard to truly prepare to these exams apart from the ones you mentioned.
Regarding the English bit - remember that passing the Wizz exams is the easy part. If you go through the selection successfully you will need to study for 17 months and pass multiple exams - all of it in English. So either way you will need to have good language skills, sooner or later
But the exam is not very hard, B1/B2 level I'd say