This boss is encountered in Knossos Palace in act 1. It's not in an optional area like the Hydra, you'll have to walk past it to continue with the story. He's there in every difficulty but you'll obviously have to beat him on legendary difficulty to get this achievement.
It shoots fire from it's left hand, and has a stomp that does lightning damage. He also has melee attacks, and they can stun you. I went at him as melee in epic and got dumpstered by his fire spray and lightning stomp, so if you're melee you'll want fire resistance, lightning resistance and stun reduction. However, the stomp and fire spray can be dodged if you're quick enough, so if you feel you have the reflexes you don't need to max the resists.
If you're ranged, you can just stack fire resistance. If you stay far enough away he'll just keep using the fire spray, and you'll never have to deal with the stomp or his melee attacks stunning you, which makes the fight much simpler.
The achievement will pop when you defeat him.