Bullies are somewhat “inside cowards”. They bully only those who are scared of them. Stand up to a bully and, even you loose a fight, you’ll be left alone and will win respect - bullies needn’t trouble. If you win - even better. But if you stand - you loose anyway.
Fight back. Bullies don’t like getting hit.
Even if the bully wins the fight, they will find someone else to pick on in the future.