Football Manager 2023

If you want to get this achievement quickly you can use Forest Green Rovers. Their first league match is against their rival so if you win that you'll unlock the achievement. Just skip to the first match and then save and reload until you win.

Go to Club information in the right hand drop out menu (LT), click on Information, then expand the >More Info< window.

There will be a 'Fierce Rivals' section..

Beat any of these teams for the achievement.

There is usually at least one team that is in your division, so if you don't beat them first time, just reload and try again.