Tainted Grail: Conquest

This will come as you play, there are 3 enemies in this fight

2 will guard the main boss. (Bit not deal dammage)

As you slay a side enemy, it gets replaced by one that does damage!

Once both the side enemies are killed / replaced, the boss get's vulnerable to dammage.

To obtain the achiement, slay the boss, the side minions can survive.

If you want to carry on in your run, you do have to kill those side enemies as well.

What I did in the fight:

Block / stun the main boss as much as possible to survive myself.
I just brought the 2 side enemies health down at the same time (even killed them in the same turn)

Full dammage on the boss.

Then I got killed by the side mobs.
It might be better to kill the sides 1 at a time, but not sure, it'll depend on your deck & buffs.