An A* is not out of the question at all considering you got a B in your mock.
If I remember correctly, I got a B/C in my Spanish Mock in January and bumped it up to an A* for GCSE.
Likewise, for Maths I received a C in my January Mock but I managed to bump that up to an A* for GCSE (lol saw the opportunity to brag a bit, but at the same time it proves that it isn't too late and as along as you're willing to put the work in, you can get the grade)
If you really want that A*, you're going to have to work hard for it, but it's definitely possible if you are determined.
For the listening, it can be hard understanding people so just make sure you listen to a lot of different types of listening.
I used BBC Bitesize for listening as they have a lot of questions and tasks relating to Spanish Listening with lots of clips for you to lisen to.
It's going to be quite likely that you won't be able to understand everything that is said in a listening exert so what I usually did was pick out key points.
Read the question first and look for what it's asking. Is it talking about holidays? If it is then look out for key vocab in the listening that relates to holidays and listen to points around that.
For the reading, it is largely vocab and grammar based.
For the vocab, I used 'Memrise'. It's an interactive website for courses to help you learn vocab for subjects such as Spanish and there are courses tailored to certain exam boards so you can ensure you are revising all the relevant stuff.
You could use Memrise for grammar too, but I don't think I did.
I used the CGP book to help me with my grammar and it has a lot of nice exam tips regarding reading and listening, it tells you ways of avoiding certain mistakes, what to look out for and what to include. It has quite a lot of nice examples and it definitely helped me in getting the A*.
Last but not least, PRACTICE PRACTICE PRACTICE! Past papers are one of the best ways to do this.
They tend to ask the same style of questions a lot of the time with the occasional different type to throw you off.
If you keep practicing you'll know how to approach the question/what to look out for and how to answer them.
I didn't do this, but a nice addition would be to read the examiner's reports. They are a useful resource and they comment on the questions answered by students i.e. mistakes, what were done well, if things were mistaken and why that could be etc etc. You can learn a lot from them.