A Wonderful Journey Reincarnated as a Slime

I'm a newbie, so please forgive me if the writing isn't very good. Here's the synopsis:

"You brat, I'll definitely beat you next time!" Veldora shouted.

"Okay, okay, this is already your 5,612th challenge," said some outsider.

"Forget it. We're all friends, after all," said a certain slime, then recalled running beneath the sunset that day. That was the youth it had lost...

This novel and its characters are entirely fictional. Any resemblance to real people or events is purely coincidental. Do not imitate.