Your "magic lantern" idea is a bit contrived. A perfectly ordinary Everburning Torch will do the job.
Continual Flame, cast by a Cleric, is a 3rd level spell with the Light descriptor. Its light automatically ends any lower level Darkness spell it encounters.
It will also take down
Deeper Darkness", which is also 3rd level, though it gives it's "life" to do so.
But if it's prepared using Heighten Spell to make the
Continual Flame a spell of 4th level or higher, then it takes down every standard
Darkness spell/effect in the book, even the huge area version cast by a Black Dragon.
Now, where would an adventurer encounter such a marvelous bit of magic? In any town that sells adventuring gear (in other words, it's right out of the equipment section of the PHB.)
To put it more simply, their tactic should have been failing, regularly, almost from the start. It should be a non-issue.
And here's the best part: Does the adventuring party have any of those among their own members' gear?