I stammer, not too bad, but stuttering in front of people in situations where it matters is my biggest fear. Partially, because I don't know how I should handle it.
That is where your advice would come in.
What would be some good comebacks in these situations?
1.) You're talking, start stammering, and other guy starts mocking you, imitating you.
2.) You are talking, start stammering, but it takes too long so the listener says: ,,Just say it already."

It should be humorous or just savage. E.g. when stuttering to a girl, it's really nice to say after it:,,Oh I stutter sometimes, but it's good, because even when I want to give 1 compliment, I give 2." It eases the tension, because you acknowledge the fact you are stuttering, and always makes the girl laugh. It's not awkward after it.