I'll bet
When someone says something that sounds like a challenge, you say I'll bet like you're daring them to lose.
You say you can beat me in a video game. I say I'll bet. I beat you in 2 minutes.
He says he can beat me in a race. I say I'll bet. He walked the whole way.
She says she can beat me in a quiz. I say I'll bet. I got all the answers right.