Lots of attemps, but no luck. Two assassinations are really hard to obtain. My current team:
Kazakus
Maestra
Scabbs
Valeera
Thrall with Doomhammer
Medivh with Unstable Runes
Medivh is my boss solution. He will counter all spells from the last survivor. 
I am curious to see if that will work for you. Because this achievement is bugged I’m a little skeptical. It is unclear that the counter from Medivh will be a solution to this. Technically if the ability triggers the counter will work but after the ability has been used. I’m not saying it will or won’t work, I’m encouraging you to prove it to us.
I just did it with heroic boss. Seemed much easier for me just because there are more heroic tokens on the path (bigger chance of getting more assassinations).
So I’ve got 4 heroic tokens on my lucky run and got 2 assassinates from them, blue boss stayed alive(1,4,7 one). He used 6 speed skill 3 times in a row and died right after.
My team: Valeera, Maestra, Kazakus and Scabbs ( yeah just 4 )
PS. Maestra changed her role in a final blow due to combo and I still got the achievement.
GIGACHAD story, even the bugged achievements shall be completed.
I just did it in heroic without Medivh. 4 heroic tokens did the trick.
My team: Valeera, Maestra, Scabbs, Garona and Thrall.
Had so much luck that thrall got a windfury treasure for the team. So the last survivor (the protector one) was a one shot with thrall, valeera, scabbs and maestra in a windfury attack combo