Medusa is located here (
https://prnt.sc/nGyKTyrvWtQ5). You need to decapitate her by using X button attacks (controller) only, and while she's on her first or second of her health bars. A big part of this is luck based, so if you have a means of backing up your save, you should do so prior or use an Inn Save made before travelling here.
I'm playing as a Thief, so I used the thief's cutting wind spam X/SQUARE attack while holding onto her head. To prevent her dying early and my pawns being petrified, I gave them the WAIT command while outside to alleviate the worry. It might seem daunting fighting this boss alone, but since you're going for the RNG decapitation anyway, it's not as bad as it sounds. Just make sure you only attack her head, and only with the X/SQUARE button. If you do this wrong, you'll decapitate her (marked by a slow-motion kill), but you'll get a Withered Medusa Head instead, which simply isn't strong enough to petrify Medusa nor is it sufficient for the achievement.
I recommend laying a portcrystal here too, as you'll need to return here two in-game weeks later for the petrify medusa achievement anyway. Don't worry, you can always pick it back up. If everything goes wrong, just rest at Backbattal (spelling?) for two in-game weeks (I recommend purchasing the property there if you have the coin) to respawn her, whether it's to then get the petrify achievement or to farm her heads.