Do not hit unless you absolutely have to. But when you do, hit hard. You should never go looking for a fight, but if someone backs you into a corner or if you have to defend yourself or others, do it. And don’t hold back, because of you only gently attack them, chances are you’ll only make them more upset/come after you harder. You only have one chance to surprise them. Make it count.
If you don’t ever defend yourself, or you never stand up to that bully, they will continue to bully you. But worse than that is the lack of self-respect and self-confidence you will develop. If you do stand up and fight, you might get beat up/lose. But the harder you fight, the less likely it is that the bully will continue to pick on you. They will want to go after easier prey. (Stand up for them too.)