Okay, ran tests (didn't take long). It seems you cannot place a Shockwave on a restricted-walk area. At least I couldn't on the dirt path between Hades6 terrain that goes up the side of the volcano.
The HG unit does work however. Sends Circe flying in a great arc into the lava (one of the Hades terrain that is walkable), over the Hades7 (the right orange terrain that isn't walkable). Problem was that it didn't kill her, so I had to lower her hit points to 1 right before the HG appears (same trigger). I also had to increase the HG hit points or she killed it right away (that's one nasty princess!).
I stopped Circe from running away by putting the HG in her way.
Still not as good as having Gargarensis picking her up and tossing her, but it does the job. Although I do like the HG unit better for looks, sort of more lava-monsterish.
Glen