I've been playing through The Watcher in the Water quite a bit over the last few days and while I really like the art, flavor, and mechanics of the new scenario, I really feel like it does have certain flaws that make it prone to being too easy.
-You can discard an unlimited amount of cards to pass the Doors of Durin. Phase two will only last one turn in all but the most unusual of circumstances, so it doesn't make sense to NOT discard your entire deck.
-You don't have to defeat any enemies. Tentacles can beat on you all day as long as you can survive 1 or 2 rounds of attacks, you can simply willpower overload your way to victory. Frodo's ability is key in taking damage from attacks or from the Watcher's triggered ability.
This is a problem because you can simply stack willpower up the wazoo and literally sprint by the Watcher. For my solo deck, I have been using Mono spirit with Eowyn, Frodo, and Eleanor. Throw in as many of the 2 cost allies with high will power (West Road, Arwen, Escort for Edoras, Riddermarks Finest, Eomer…) along with certain events (Stand and Fight, Test of Will, Hasty Stroke, Shadow of the Past, Astonishing Speed), and you have got yourself a deck that can pretty much never lose, as long as you throw in a couple cards that contain the right letters and can also help with Willpower (Bofur, Gandalf, The Galadrim's Greeting)
With Eowyn, Frodo, and multiple allies questing, there should be no problem beating phase 1 in 2 turns. Once you are on phase 2, simply discard your entire hand so that at least one of the cards matches the encounter deck. Hopefully you have a Stand and Fight, so you can summon an Escort from Edoras, but even if you don't, it shouldnt be too hard to make 5 progress to beat the quest. Use Frodo's ability to take the Watchers 3 damage, or simply kill off Eleanor since there really aren't any truly bad treacheries in this deck and you won't need to defend once you've won the scenario.
Thematically, I understand that to kill the Watcher should be much harder to beat the scenario than to simply pass the door, but I wish there was some additional mechanical reason to engage the Watcher.
This will also work for two player- I've been using a Mono Leadership deck with Prince Imrahil, Aragorn, and Theodred to go with the mono spirit from before. Simply power quest your way to victory, Imrahil and Aragorn can ready themselves enough to defend the attacks as necessary from tentacles, wargs, and uruks. The only way I have real problems is if A Bitter Wind comes up during set up.