Diablo3: How to do Bashanishu Achievement

This will tell you how to complete the achievement Bashanishu.

Heading to the Shrine of Rashanishu in act 2 at the Dahlgur Oasis an event will start that gives you the blade after completion. There you will find Rakanishu's Blade. Bashiok will also be in the area and you must equip the the blade and do damage to Bashiok. You do not need to melee him down the the blade the whole fight just doing a swing will be enough.

Bashiok on the other hand is just a rare spawn monster that wonders around the oasis. people have found him in all difficultys. The event will not always be in Dahlgur Oasis when you check, so just restart your game and try again if you cannot find him.