One way you can make it easier is to start with a compliment or something positive:
“I know you really care about me and…”
“You always say the most supportive things, so maybe you didn’t realize X thing was hurtful…”
Then you want to make sure you clearly set the boundary:
“When you call me a big back it really hurts my feelings. Stop calling me that.”
Or:
“I told you that when you call me that it upsets me. If you keep calling me that, I can’t be friends with you anymore.”
Sometimes, if someone is not respecting boundaries or hurting you or others, it means they need to learn more about friendship before they can be a good friend, and you might have to spend less time with them or not spend time with them at all, because friends don’t hurt each other and friends respect each other’s boundaries.