Penny Pincher achievement in Baldur's Gate 3

The Toll Collector or Gerringothe Thorm can be found in act 2.

Enter the Toll House (don’t worry the skulls won’t attack you) and on the top floor of the you’ll find The Toll Collector.

The way I handled this was to allow her to take all of my gold (though I’m not sure if this is necessary), wait for her to walk close to the opening in the wall, save, and then enter turn based mode and use thunder wave to send her flying outside.

She should either outright die, or be essentially one shot (I had to hit her with an Eldritch blast). If done correctly the achievement should pop.

The Skulls should disappear when she dies

There are a few other ways though!

You can kill the skulls, preferably from stealth, and for each skull killed you’ll weaken her. Kill enough and she’ll be exceptionally weak.

You could also polymorph her, then kill the skulls.