An attempt at making The Fall of Gil-Galad work.
The Fall of Gil-Galad needs to be placed on Glóin. Once Glóin leaves play, the threat will be below 20 provided Galadriel has been keeping the threat low. Fortune or Fate could even bring Glóin back later in the game, if required (and if you can draw it!).
I have two cards supporting secrecy - Swift and Silent and Ithilien Lookout. The only hero that realistically could leave play would be Glóin, and his resources need to be aggressively spent.
This is a poor deck. However, it can be made to work (with some patience). The key card is Mirror of Galadriel - this allows fetching Steward of Gondor (to put on Galadriel), Heir of Mardil (to put on Théodred), The Fall of Gil-Galad (to put on Glóin), and Brok Ironfist to pull out when Glóin is gone. Plan B if The Fall of Gil-Galad is too troublesome is to utilize Gandalf and The Galadhrim's Greeting to lower the threat below 20. This deck really relies on getting a large hand to begin with, and then using Mirror of Galadriel extensively to fish for decent cards - in long games the deck can get rather depleted.
Modifications I'd consider include ditching A Light in the Dark and an Orthanc Guard in exchange for Will of the West (to avoid having a deck full of leftovers, and to allow more aggressive use of Gandalf), and including more allies such as Orophin. The Son of Arnor and A Light in the Dark combo is cute but I have yet to make that work, so those would be candidates to ditch. Having Saruman would be great, but the doomed 3 might be rather steep - getting an initial low threat can be tricky. Getting rid of Glóin in exchange for a threat decrease largely seems unnecessary - Brok Ironfist kind of makes up for that, but overall this might be better as a secrecy-orientated deck. Valiant Sacrifice might be a good place to start for early-game card draw, but alternatives exist such as Deep Knowledge (this requires Nenya though, which is not guaranteed although rather useful in the early game).