Successfully completed the fight without using minions and without any of my characters changing roles and the achievement didn’t pop. The opponent’s roles changed but there isn’t anything that can be done about that.
First time I attempted it, Shade of Aran used an ability to change my role, which is pretty unfair for the achievement.
Same, I fulfilled the criteria, and didn’t get it.
Same for me. Used Chi-ji, Barron Geddon and Deathwing, since it’s easy to just spam 3, 6 annd 7-speed abilities respectively, and just skip a turn if you are slowed down to the wrong values. Plus, Aran will always target Deathwing with his ability that would change any nonprotector to a protector, so you are safe from that. So I am 100% certain my roles didn’t change.
Same for me. I kill the boss in the 1st turn with super buffed orcs. Got so ultra lucky. My Valera snipped the protector instantly away with the rare treasure. All my orcs could use their 5 ability unhindered. But the achievement did not trigger. Yeah its bugged.
The achievement actually requires you to defeat the boss without the enemy changing anyone’s role, including the boss’s and his assisstants’ roles. This is impossible without extremely lucky treasures, such as two copies of Assassinate 2, along with a Prideful wish that eats the last enemy.
I can confirm, tried to do it yesterday for 2 hours, never got lucky with the elite treasures or if I did, the remaining non-assassinated boss/es were still able to change their roles.
Bugged for sure.
Can confirm 05.10.2022 EU Server achiev is also not working.
If so, which mercenaries can get Assassinate?
I know those can get it:
Valera
Garona
Maestra
Snips
Bwonsamdi (eventually)
Got it today with two assassinations on Scabbs and Valeera, killed last enemy manually without triggering his role change effect. Got achievement.
I don’t know who is Snips ?
what team did you play?
Just an update to this bug. It seems as though the enemies effects triggering at all (Even if no role was actually changed, ie protector to protector, fighter to fighter, and caster to caster), will disqualify you from the achievement. https://imgur.com/a/jOSXL51
This may also mean that it’s technically possible to get the achievement if the enemy roles change, so long as you can avoid their effect from triggering for your own units, but I haven’t yet tested this theory.
Update: After testing it I can confirm that enemies changing roles will disqualify you, even if absolutely nothing changed your own roles, even to themselves.
Wow you are really lucky. I try it over several hours but no luck to get two assassinations.
My team:
Garrosh (Orcish Banner)
DrekThar (Glacial Blade)
Thrall (Doomhammer
Maestra (Mask of Mimicry)
Scabbs (S7 Contract)
Valeera (Unnatural Smoke)
Yeah Snips is the german version. 
It`s Scabbs.
Just a lot of attemps. If you didn’t get any assassinations in the firts three encounters, reset the bounty.
My team was: Valeera, Scabbs, Vanessa, Kazakus, Old Murkeye and Murky.
Also finally got it with
Got 2 assassinations. Actually got 3, but the third was vs. neutral.
I am having extreme trouble getting this done. Even when I got two Assassinates, the Fighter Merc always casts Shifting Strike, a 3-speed abiltiy, and turns herself into a caster.
Well done men. Rejoice, for I have also figured this one out. I will try to add some kind of strategy to this topic that may be able to help others!
The build I chose was:
Kazakus, Maestra, Scabbs, Valeera, Holmes, Sir Finley
The reason I chose these mercs is they all can potentially receive the Assassinate 2 from heroic token encounters.
While trying to do this I would never see Assassinate 2 from the normal token encounters. So my strategy was to at least use a map that had two of the heroic token encounters on the way to the boss.
After many attempts I was able to find 2 heroic tokens that gave me Assassinate 2.
Upon encountering the boss I concluded that if the RED boss did not die then it was impossible because he uses abilities that cause him to change his own role. The BLUE and GREEN bosses have a chance to not do this. However, the BLUE boss has the best chance to not use his own abilities in my experience.
When I engaged I was lucky enough to have the BLUE boss as the one left standing. He cast a 6 cost ability twice which did not proc his 1,4, and 7 requirements. I made sure that none of the abilities I used to win also were not a 1,4, or 7 speed either.
I received the achievement for the hard fought struggle.
The End.
You played heroic or normal bounty?
Normal Bounty. Heroic tokens within the normal bounty are what you want.
Although it looks like the Heroic Bounty is ok as well as long as you can guarantee 3-4 Heroic tokens for the run.