I've been trying to get the "Bashanishu" achivement pretty much since I got the game. At first I was like "meh he'll turn up"...boy was I wrong. So I had some runs at it, I tried to remember taking the blade with me every time I got to the Dahlgur Oasis and I already played with all the characters so I've been there for like 20 times. Not even close...
After hunting achivements for a while I looked for it in all kinds of forums to see if there's a guide or something I'm doing wrong. Turned out it just the most annoying achivement known to mankind. Grinding doesn't even start to describe it. Today I said enough! I was at it since noon and finally got it 10 minutes ago (the time now is 2:30am around here). I only had 2 breaks (lunch and dinner, not counting the bathroom breaks haha).
So if anybody wants to get this achivement - brace yourselves.
The blade is really easy to get, it drops every time you complete the Shrine of Rakanishu event on NORMAL DIFFICULTY ONLY. after that...it's all about luck, persistence and will. that little bastard can spawn at any place around the oasis which is a pain cause it's a relatively large map. Plus, it can spawn with or without any other event or random spawn that can happen there, so yeah...you'll have to open the entire place if you want to be sure. When you finally got Bashiok be sure you equip the Rakanishu's Blade AND don't have any skill on your normal attack. One hit will give you the achivement, but you'll probably want to take your frustration out on the guy so don't hold back.
I used the barb so i could use the sprint to makes things a little quicker, I guess the monk would be good for that too.
I read somewhere that he spawns closer to the Path to the Oasis waypoint so I started there every time, but found him somewhere in the middle of the map so I wouldn't count on it (maybe it has a higher probability to spawn on the south-west side I don't know). Anyway, I finally got it, and also got the "Shut Up and Take My Money" and "Champions Collection" (I was missing a spirit stone I got from Javad and killing a Deathly Haunt in the Mysterious Caves), so in conclusion - today was a good day. and now for some sweet sleep.
I hope this was helpful to whomever is going to try this, and I'd love to get some comments.