Fairly do-able even on Honor Mode if you use illusions or a bard playing music to lure every witness in the room away, and also shove the flying eye into the abyss (and then leave/go invisible before someone comes to investigate why the eye “”“died”“”).

Then if you surprise her, use the surprise round to just (non-lethally) cave her head in with a raging barbarian using a maul or something like that, then proceed to wipe out the rest of the leaders, once you report back to the grove and finish up Act 1, she will be in Act 2 getting scolded for failing.

All that dialogue can also encountered without knocking her out. The trigger for it is to completely ignore the Goblin/Druid conflict. Without your interference, the Tieflings are cast out, the Druids finish the ritual and magically seal off their grove, Minthara gets reprimanded for failing her objective, and you have the option of saving then recruiting her from there. Doing this means you miss out on Wyll, Halsin, the breadcrumb for Karlach, so they give you Minthara.