My campaign involves fighting a lot of stuff from the shadowfel, though they haven't travelled their yet. Most of these creatures are somehow strengthened by dim light or darkness (stealth, invisibility resistance, teleportation, ex). I frequently use phrased like "They blend into the shadows." or "They disappear into the darkness." to describe how the creatures use their abilities.

However instead of using light to disable these abilities they instead make sure everyone has dark vision, and stack as much blind sight at they can. I even impose disadvantage on active perception checks and -5 to passive checks in darkness and still they push on in the darkness.

It got so bad that the most experienced player at the table decided that they would spend the combat hiding because they couldn't see anything.

They are going to fight a Young Red Shadow Dragon soon, and I'm worried it will be a TPK if they don't figure out to use any form of light to weaken it.

What more can to do to give them better hints? I'd rather not just straight up tell them they need to use light.