Getting a 50 in any subject isn't easy, and there's no real method to getting one. With spesh, you've really got to be perfect in everything you do since it's such a competitive subject. I've seen many people who deserved really high marks and even 50s, but they didn't get them because they weren't on point, even though their knowledge was great.
Methods and spesh are different - you'll need a very high theoretical understanding of the material in spesh if you wanna do well, not so much in methods where you can get away with memorising formulas and questions and plugging in numbers. Just keep constantly revising and tackling as many different questions as you can for each topic you do, which should get you thinking about how to apply the theory in different situations.
Extra reading would probably help if you're really keen, just to help you get a deeper understanding of the material. I've really gotta stress the theoretical understanding of the maths, because there are so many little things that they can trip you up on and some situations can be really foreign.
Once you've got the knowledge though, it's all a matter of perfection, like I said before - setting your working out correctly, making sure you're not leaving anything out, your writing is legible and logical to read.
Honestly, spesh is hard enough as it is, you've just gotta keep your fingers crossed that you're going to have a good day come exam time and don't screw anything up. I think it's probably safe to say that the people within the 45-50 SS range all have the same/similar level of knowledge, it would have just came down to whether they had a good day or not and managed to be perfect.