What should be the best strategy in this regard is something which belongs to the domain of sociology and psychology.
As far as Islam is concerned, Islam provides an ethical framework build on innate morality which is ingrained in our nature. What we can do is give voice to the same innate morality to the one comitting such acts. And if that person is a Muslim, then Islamic references can also be used. For doing this, it is better to use an appropriate medium which not only conveys the complete message but also does so by not provoking the ego of that person (e.g. through a family member of that person).
If the person does not respond to this measure and keeps on committing such evil acts, then instead of working in favour of that person, attempts should be made to help the oppressed in this scenario. This could be done by equipping him/her with the tools to counter bullying, creating an open environment for the person to bring his/her issues regarding bullying and not hide them, involving the parents, families and society to fight this evil as one voice, raising social awareness through formal and informal education means, in some scenarios even standing up to the bully on behalf of the weak etc.
Whatever strategy one chooses to follow, as a Muslim we must always uphold the principle of justice and fairness.
Follwing thread contains some videos which explain different strategies in dealing with bullies:
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