Baldur's Gate 3

You do not need to start a new character for this achievement.

As soon as your party reaches level 12, pick a hireling from Withers, then go to level them up and give them one level in each class.

This will get you your achievement, you can then reload a save to get back the money you gave to Withers.

The perfect place to get this is at the Elfsong Tavern basement in the Lower City, Act 3. There you´ll find more than 20 rats with 1 hp, all together waiting for your preferred aoe spell. I used Ice Storm and got it in inmediatly, but any spell with a big area will do.

So just take a drink and go wild on those little pesky rodents.