in the patch notes it says:
* The Rare bosses spawning in the Tarantia Common District as part of the riot have been changed to use non-elite stats so that they can be killed without needing a group.
I just tried now, and the boss died almost instantly, but i was not awarded the kill.
I Guess my mistake was not killing all the rioters and they tagged the boss before i could. But then again i fear the boss as a non epic will not manage that.
Anyone that have gotten the achivment update after the patch?