You want to win ex boyfriend back because he was yours and for some reason she has him now. If you believe that the two of you should be together then it is very likely that you are. But how are you going to do this? You have to play it smart and dignified if you are going to win ex boyfriend back.
She may want to play nasty to keep him away from you but if you get nasty right back then all you will be doing is feuding and you are too big for that. What you want to do is show him that you are more mature than she is and more like a real woman while that other girl is immature.
If your first instinct is to fight, suppress it. Rise above it. Gain control of your emotions and the other girl may be exposed for being the immature girl she is. If you look favorable compared to her then it will only be too obvious that you are the better choice. Rather than resorting to curses, resort to positive feelings. Be the obvious better choice and you stand a greater chance to win ex boyfriend back.
If she treats him poorly then talk to his friends. They may be feeling the same way. If it keeps going you may have grounds to have an intervention. Talk with their friends and find out with them what you could do together to get him out of that situation. If you want to win ex boyfriend back it will be easier if you have his friends working with you. Much of the best ways to win ex boyfriend back from her is going to be using his friends.
If it seems like he is under some love spells that she put on him, find out what it is that she has over him and try to find ways to show that he deserves better. Let his friends know that he deserves better. They may very well agree with you. If she has him obsessed with her and she is forcing him to spend time with her instead of them, they will want to get him back. If you are on their side, they will be on yours.
If you are able to convince others who know him well that their friend is better off with you then you will have a lot smaller of a battle to fight. There will be so many people against that relationship that it could crumble under the pressure. The secret to having this work in your favor is to be behind the scenes. Let others do the work for you. That way if that other girl catches on you can make her seem like she is paranoid for thinking that you are causing all their problems.
The key to getting what you want, which is to win ex boyfriend back, is to have as many people on your side as possible. It doesn't have to be you against the two of them. It could be the two of them against the world. When their relationship falls apart, you will find it easier to win ex boyfriend back.