Images are from my map guide to the game. This is not an ad. These are just the easiest images I have to illustrate the guide with.

Fight them as a boss encounter.

Find them in the Hotsprings on the way to Volcanic Valley of Guenna. Be warned there's some vaguely creepy art in a cutscene here.

You'll find them in the same room you had a lock-in challenge before. Fight off some demons to literally rescue one of their butts.

Find the sisters at the top of the icy mountain. Take the opportunity to do another achievement while you're there.

You might have noticed this odd ledge the first time you made your way down through the Forest to the Cursed Cavern. Now you know what it's for.

In order to get to this corner of the Cursed Cavern, you need to freeze the water in the Crimson Sands of Elug connector room above and drop down.

Use one of Misha's freeze seeds and chuck it just at the edge of the land so the explosion freezes the water (or use an appropriate subweapon if you can get one to work) and then destroy the ice with your weapon.