I came up with an easy (well, really not quite as hard) strategy for Zephyrine. Basically, take your party to some far away corner of the map; the ogre stronghold in the lower right works well because of the gates. Enter combat, and just send one of the party members to Zephyrine. I used Shima, but anyone would work as long as you have a way to teleport (skill or scarab). Make sure that the party member by Zephyrine is the leader or first member of the party.
Then, end combat and talk to her to start combat with her. Teleport as far away as you can and run/teleport back to the rest of your party. Zephyrine will follow you, and since you haven't damaged her, none of her guardians will be active. Once she gets into the ogre stronghold, close the grates and pull her away from the grates.
Now you can kill her at your leisure, and her guardians will activate but not be able to get to her and heal her. Be warned that once you do kill her, the guardians are still roaming around looking to kill you, so you will have to fight them. If you're lucky, you should finish Zephyrine before they're at the grates and be able to escape. Also, the guardians aren't nearly as tough without Zephyrine hurting you.
One thing to note, is that in doing this, I did not get the dragon tooth until I went back into Zephyrine's normal area. When I did, I then got the message about pulling a tooth. I'm currently doing a singleton playthrough on torment, so I'm going to try this method once I get to level 30 to see if I can do it with only one character.