If you need support with a subject, ask for help. Even if you don't have the best rapport with a teacher. It's better to ask for help than to let stress consume you or to continue not understanding something.
For languages, the best revision technique for oral exams is to practice every question from every past paper you can find. Practice answering the questions out loud in your target language without preparing an answer for them to see if you can talk for a sustained period without preparation. This definitely helped me.
As for extracurricular activities, I think it depends on your time management skills. I was able to do 4 subjects and enrichment during my A2 year but I couldn't have swapped enrichment for a part time job as I needed my weekends and evenings to focus on homework/revision.
Lastly, I wish I'd known which subjects were considered the most important for the degree I wanted to do at the top universities. I definitely paid for not consulting the universities prior to AS.