For this achievement you must defeat Medea.
This Achievement is story related so you will not miss it. Medea is located on Level 29, Lost Fortress. If for some reason you are having trouble locating her make sure your playing the right game. If your worried that you might not be able to defeat her then consider these following tips.
-Prior to the battle make sure you have plenty of health and mana potions. Can be found at the general goods merchant in town or as loot just about anywhere.
-Check to make sure all of your equipment has recently been upgraded. If not use a town portal scroll and make any last minute adjustments.
-If you are unsure about your experience level you can always purchase map scrolls from one of the vendors in town. These are good for gaining any experience you may be lacking.
-Set your pets mode to attack instead of defensive or passive. Should be the first option from the left in your pets menu.
-Make sure you have a plan as far as the three options given upon death (i.e. pay gold, lose experience or be sent back to town).
Unless you are on a higher difficulty you should have no problem defeating Medea.