Rajani is the first major boss that you can use for this, and probably the easiest. In order to do this, you need to be playing as either Anjail and Katarina, and have your companion be the other. It also needs to be co-op, either online or offline. Dapper Old Gent and the final boss are the only other bosses you can do this with, I believe.
For this, I used Katarina and had Anjali as my partner. I did it local, without having a profile signed in on the second controller, on casual difficulty. I let my companion idle during the fight so the AI took over, then right before Rajani died the second time, I pressed a button on the second controller to unidle her.
Achievement unlocked. I'm not sure if forcing her back to manual is necessary, I did it just to be safe. You might even be able to just leave her dead, but it'd be quicker to have her attacking too.
Zulghinlour said that his AI was dead and he still got the achievement.