Anyone know exactly how final sting works?

Ran Temple of Qarn last night for the first time, we made it through however I was randomly one shot by final sting on the bee trash multiple times.

Tried burning it down fast, tried cc'ing it and saving for last, tried watching the bee for a shield stun but didn't see a cast timer, it would still randomly get off final sting though.

Interesting thing is it looked like final sting actually did less damage the more hp the bee had left, I had one go off at high health and it only hit me for like 1200. This compared to the first one I took where it insta-gibbed me for 2500 damage and made my healer go wtf?

Anyone have some insight into the mechanic and/or a reliable way to deal with it for future runs?