I ran through Rkindaleft yesterday and killed all the bosses, including the group boss, while solo. The game didn't acknowledged the group boss, though.
I was alone and killed each mob after activating the respective chamber - activated chamber, killed mob, moved to the next until all 4 were active and I could fight the boss. I didn't die, didn't get out of range and didn't exit the instance while doing this.
I've solo'ed other group bosses before with no problems for the achievements. Is this a bug or did I do something wrong? In case it's relevant, the order I activated the chambers were:
1. right of entrance, front
2. left of entrance, front
3. left of entrance, back
4. right of entrance, back