Baldur's Gate 3

At the end of Act 3 in the final stretch of the game, you will return to the Astral Prism before going to the Upper City.
There are couple of arenas to fight through before climbing up to the top of the Netherbrain.

This is the final fight which includes a mind flayer controlled Red Dragon which you just have to kill to get the achievement.
You should be max level and geared up by this point so the fight isn't anything complicated. You don't need to kill the dragon to complete the fight but there is a round timer until a mind flayer ship comes in so focusing the dragon prior to this makes it slightly easier.

There are 4 fairly powerful creatures that start with the dragon but, depending on choices throughout the game, you should have a number of reinforcements to call upon which it even easier. There is a spell caster enemy that likes to counter spell alot so you may want to focus him first if your relying on that.
I'd recommend hanging back and holding the area you spawn in as if you move through the area brain vines will spawn and there are 4 mind flayers in the back that spam force missile which can tear through anyone.

Hang back, spread out, call reinforcements and kill the dragon to pop the achievement. Then your free to pick your way through the rest of the enemies.