How to win a tickle war against this girl?

This girl/ coworker I know at college found out I was ticklish. She now won't stop trying to tickle me. I got her once and tried to prove my point. She just keeps saying she's going to get me. I absolutely hate being tickled. How can I win this war against her? And why does she want to tickle me so bad?
How to win a tickle war against this girl?
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