Tiamat is one of the first bosses you meet in Darksiders, in true Zelda style you will find a new weapon in her dungeon that you will need to beat her.
The fight consists of throwing bombs at her as she tries to set you on fire (dodge this) and then using your new toy to target the fires surrounding the arena and the bomb attached to her face.
When it goes off she'll fall in front of you where you can hit her with all you've got, repeat this (and also the dodging) until she goes down, you rip her heart out and pop the achievement.