If shovel can trigger it and spawn them while Tav is in stealth, it should work as an ambush. I don't know if it will actually start the fight for shovel, though. Or if OP is fine summoning shovel in a solo playthrough.
I only know for sure that at least sometimes during patch 9, you don't need to talk to the tiefling. I had one of my party accidentally step in the water while experimenting with that fight and it just instantly summoned them.
Combined with fly at level 5 + barrelmancy before they even spawn (should they truly only spawn when you step into the /water/) = cheese?
Infinite invisibility loop is there, too, but that's expensive. And annoying. Plus, you could be charmed start of cutscene. Using shovel can avoid that (?), but only once. Still have to cheese or face solo game over for being charmed.
