It looks like you had to have 1) the right weapons to pull that off 2) were within a certain range of levels and 3) get a specific type of boss spawn. I pushed a new build to safeguard against this in future, so I'm fairly confident you won't run into that again.
Anytime, I actually really love this game and I'm glad I got the chance to play it, will play it tons more. Specifically, it was the short range, somewhat wide laser, I think I had it on both Primary and Secondary and fired as fast as possible on the boss.
