Yeah... spent the last 2 days on Taris and no sign of this guy.
Sad part: I've killed him at least 3 times that I remember in the past before the achievement system. Now he doesn't spawn and my past kills don't count.
Please fix his spawn rate Bioware. Standing around for hours waiting for a monster to spawn is not fun.
Finally got him!
Logged out yesterday in front of his spawn spot. (He wasn't there.)
Logged in this morning at about 09:30 GMT. Still not there. Checked and only one instance of Taris was up. Waited and waited. At about 11:45 GMT a new instance of Taris was spawned (the current instance was at 116 players when I noticed the new instance). Switched to instance (2). He was there! Killed him. Dinged the achievement! Did a happy dance and logged out.
It is confirmed. He must only spawn once per server reset/instance "creation". The trick was waiting at his spawn spot until a new instance is created for Taris and then switching to that instance to catch him.
Hope that helps others waiting/wanting to kill him.