… You have 2 boxes to open. You do 2 fast (no killing involved) quests while waiting for RF to pop. In that RF you get kill #20 AND Fatality #5 on the last boss. So you have 4 daily tasks complete and only one buff to show for it.
I spent the next 30 to 40 minutes trying to get an unchained going before I went to work, but nothing.
I can’t count the amount of times I’ve gotten kill 20 and fatality 5 at the same time.
I call that a trif***ta…oops, I mean trifecta
I’ve been in the same situation as you … and after that happened once I learnt my lesson.
If I have two or three boxes to open then I find a set of low level npc I can safely kill with white hits to get my 20 kills without risk of fatalities (even on a caster - when I’m getting close to the 20 kills but haven’t already got the fatality buff then I kill one npc at a time with spells or even switch to melee white hits). The 20 kills should be fast. The fatalities can take more effort, so I usually try to get those whilst doing a couple of quests.
Fatalitys is nice to do at the chickens in heavens lake, they respawn rly fast. Do combostarter in the air and only land finishers.
Spamming the slam instant combo from level 2 or so is a nice way to do it also, you can instantly interrupt it and start the combo right after (pressing W and the combo starter rapidly at the same time so to say) that again until you land a finisher.
That works on ranger too: start a “combo”, and if it doesn’t trigger the fatality, cancel it and try again, until you get it. 
Yes exactly, it works for all instant combos.
Barbarian’s decapitation too