Solasta: Crown of the Magister

After being instructed to kill Greybear by the elder travel to the inn, he will confront you and ask you to investigate the elder for him. Long rest at the inn. After the rest, he will not even be agro on you. Walk right up to him preferably use a high level lightning bolt on him and his friends that are all standing in line. This will give you a surprise round and an easy fight.

It is worth noting that this is the bad ending solution for this map so I would suggest loading a save after getting the achievement.