In order to crack a group discussion you will first need to understand  what skills are judged in a Group Discussion.
Here are a few skill that a interviewer will look for through a GD.
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How good you are while communicating with others?
 
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How you behave and interact with a group?
 
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How open minded are you?
 
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Your listening skill.
 
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How you put forward your views?
 
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Your leadership and decision-making skills.
 
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Your analysis skill and subject knowledge.
 
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Your problem solving and critical thinking skill.
 
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Your attitude and confidence.
 
Hence I suggest you to work on these aspects.
All the best!