I used to have a good friend local who was near my level of SC, before sc5 came out. He started off with NM, and me Sieg. It didn't take him very long to give me a run for my money, and it became very stressful to beat him.
Anyways, he got frustrated with losing consistently, and switched to Ivy. On his day 3 or 4 Ivy, I couldn't beat him anymore. He just stuffed and countered everything I did. I was literally reduced to turtling, grabbing, 1k's, and not much else.
This guy was exceptionally brilliant with natural talent. I had a huge edge in experience, though. I give the guy mad props for learning so fast.
So what did I do? I learned Cassandra. It quickly became apparent that she was a higher tier than Sieg. It took about a week before the tables completely turned around and now HE couldn't beat me anymore. He was particularly salty over 236B wall spam, which I'm not even sure was best thing I could do in that situation, but it still annoyed him and I felt like a whore doing it.