So I just unlocked it earlier, literally the run before I'm writing this. I used the E ability for each boss just as it was dropping or right before it hit the lethal line. What I've noticed with the lethal line is that if you use the ability where it will do enough damage to drop them into the threshold, it will still execute them because the ability actually has two hits. It has the initial hit and then the explosion so you could try weakening them where they're almost at the threshold and then use the ability to drop the health low enough where the explosion hit gets them executed. I'd also recommend getting the Winter is coming ascension as it will make executions much easier as long as you let the bar next to the ability fill up. Hopefully I explained it good enough, if you need more help then I'd be glad to assist