However, what I want is to try and recruit Minthara by attacking the druid grove. I want to completely wipe out the druids, but I want to spare the Tieflings and have them all leave for Moonrise before I attack.
I don't see that happening. I suspect that, even if you do it as Ecc2ca says above, storywise all the Tiefs are dead, regardless.
A thing you *can* do is this:
Knock out Minthara as a temporary hostile - eradicate the other two leaders before resting [because I presume Mithara will wake up, otherwise and will be full hostile by the time you killed the other two]. This saves the Tieflings. Proceed to Act 2, recruit Minthara.
Travel back to the Grove and kill everyone there.
Edit: I seriously wonder how that plays out. I mean, you could potentially have Halsin *and* Minthara in your party when doing that. I don't know if they have lines for when you go there. Let alone if Halsin will leave your party if you attack the Druids. He should, but, you know. I have no idea.
