Contains spoilers for Beyond the Wall
After watching the latest episode - which someone SOMEHOW found out about ahead of time and spoiled, I didn't believe it fully until I saw the episode - I have no doubt that the war against the White Walkers - even with Cersei on their side, as she will soon be convinced of their existence - is a TLC. Which in this context, stands for Totally Lost Cause.
Even though we now know killing the Night King will wipe out all White Walkers and Wights, it's an impossible task. Killing Voldemort, Sauron, Jason, Pinhead, or even Thanos - who HASN'T died yet - is easier. (And I already have a theory on how the Avengers defeat Thanos in Infinity War)
First, dragons can't get near him. DRAGONS. I thought they'd be the ones to defeat him. The Song of Ice and Fire told in the title of the books happened this episode, not the last, and it ended in victory for ice rather than fire. He ALWAYS has a spear handy, and if a dragon even started to fly at him, he'd throw the spear and kill it. We know it can kill dragons, coz we just lost one of them this way. That was worse than Ramsay killing that giant.
Second, even if a dragon could get near him, what good would it do? Unlike other White Walkers, he's immune to fire. He walked through the flames and they dissipated as soon as he got close. If Drogon or Rhaegal shot a blast of flame at him, it would never connect with him. He simply cannot be burned, because his "aura" prevents it.
So, only weapons of dragonglass or Valerian Steel can kill him. And, if dragons can't get near him without being fatally speared, no human can. Besides, even if they're wielding one of those weapons, they'd have to cut their way through hundreds of Wights and some White Walkers to get to him, and they'd likely be torn apart if they tried.
Third, as if that wasn't bad enough, he's got an undead dragon on his side now. If he wasn't unstoppable before, he is now. This same dragon is the first dragon to die - and after our death scare with Drogon two episodes ago, with that weapon destroyed - and it was the Night King who killed him. The only way to take out the zombified Viserion is to kill the one who turned him - which is the Night King himself.
I heard a rumor that George R. Martin said the only five characters to survive would be Tyrion, Jon Snow, Arya, Bran, and Daenerys. This is obviously someone lying to get people to worship him. The only character that will survive is the Night King. Possibly the zombified Mountain (who seems to count as his own character) coz he's already undead.
So, the final episode is likely going to have the meeting with Cersei for the first half, and then the fight for the second half, in which everyone gets massacred by the White Walkers. Or else end like Angel with them about to face the evil army, but with a considerably lower chance of winning.