I have developed some tools to use with bullies who are in my life and my favorite ever tool is what I have named the Ouch. You Stepped on my Toe. Back off. Rule.
The way it works is simple. Say you're in a social situation and someone says something nasty about you. You say, out loud, right in front of everyone, and very calmly,"Ouch. That was a nasty thing to say. Don't do that again." Or even, "Ouch. That hurt. Stop it."
Suppose someone says something snarky aimed at you. Those kind of hidden things can be like tiny jabs, and they can be tough, but not with this rule. You just say in a calm, firm voice,"Ouch. That had a hidden jab. Don't do it again."
If someone says, "I was just joking!"
The response? "Well, don't."
If someone says, "You're touchy."
The response, without defensiveness--"Maybe. Don't do it again."
The biggest problem with this tool is that sometimes people don't see an attack at the time it happens. We might have been trained as children to be nice, or to always assume the best, or if we were raised by manipulate bullies instead of direct ones, we literally have been taught not to see bullying when it occurs, which means we have been taught that our "ouch" response didn't ever happen--i.e. we have learned to override our own wince of pain. It might be days before we realize we were jabbed.
The ouch rule says, "Speak up as soon as possible after the event."
The ouch rule says, "Speak up every time."
The ouch rule says, "If the insult is public, the ouch should be public. Sometimes, if the insult was private, the ouch should still be brought up in public." Next time you're in a small group with this person, say, "Remember the other day when you said x? That was nasty. Don't say it again."
Special note: it's hard to teach a shy kid to do this, but it can be done. The more you do it, the easier it is. No fuss, no muss, no big drama, just bully, back off. Good luck.