How much time does the invisible lava stay? In other words, how much leeway do you have triggering the chest?
Ground effects generally have a duration, but perhaps invisible lava disappear quickly, I don't know.
With the barrelmancer thing, the Onyx and Leandra are both reliably killable (use a smokescreen on Leandra, a companion makes it easier but not necessary), but the Void Dragon can give surprises (I guess in explorer you don't care that much though) because it has the initiative. Plus, the lava trick is just elegant ^^
