How do I finish an IGCSE subject in a month?

First of all, be optimistic. You have every reason to be.

If you are repeating a subject which you have done already to get a higher grade, then we think a month is very ample time to get extremely familiar and comfortable with the subject and pull the grade up to an A* as you indicated you would like to achieve.As you stated that you have already gotten an A and was 2 marks off an A*, We think you have a pretty good foundation and understanding of that topic already so now to make sure you get that A* just requires a few minor tweaks and additions to you're already good knowledge of the subject.

In terms of these minor tweaks we feel you should cover, this is what We think you should focus on

1. Get familiar with the challenging 'non routine' questions. In other words, the ones that are put in there to scare you, unsettle you or throw you off and is ultimately what separates those who have an excellent understanding of the topic (A* people) from those who only have a good understanding (A people). The best way you can familiar with these questions is simply doing many many practice papers. Do not do them blindly. You have a month, take your time in doing them but make sure you analyse each question and its corresponding answer that the IGCSE looks for so you are familiar with the way you should answer certain questions in the best way possible. This will expose you to the common sorts of questions they like to ask and also expose you to the challenging stuff and how to solve it.

2. Know the relevant theory or concepts back to front This is important in making sure you have an excellent foundation whereby you can build upon and answers questions with the proper terminology or theories the IGCSE is looking for. It also helps to fully know and understand what the question exactly wants. To master this part, make sure you make your own hand-written summary notes on the topic so that you can extract the key info and remember it for when you need it. If this is not enough, the link here will give you a more in depth answer about note taking and practice papers too

3. Cut down on careless mistakes .  These are the shittiest, most heartbreaking, most terrible way to lose marks because you actually know the answer but were let down by sloppiness and in many cases can cost you a few marks that it affects the final grade. The good part about these mistakes is that they are easily preventible. So learn to prevent it! Do as many practice papers as you can and on each subsequent one, try and cut down the number of careless mistakes you make by half. If you want to make it more fun, impose penalties on yourself or something like that to stop you making careless mistakes because seriously, if you can make 0 careless mistakes in the exam you could gain yourself an extra 5 marks or so which could make the difference between the A and A*.