Just finished rewatching the entire series, start to finish - well, up to the most recent episode. Pretty clear that the White Walker and their Undead Army represent a major threat to humanity. However, I do believe there is a very plausible and fairly straightforward way to beat back the army - with or without dragons and Daenerys' legions of unsullied and horse-whisperers.
If this happens:
- The North unites under Jon Snow and Sansa Stark - meaning all rebellions (specifically Bolton's) must be crushed quickly
- The army of the Dead is unable to use boats and must cross over/through the Wall
- Pyromancers get enlisted to make more wildfire
- Bag of dragonglass is recovered
Then this could happen:
- Caches of wildfire placed strategically along/atop Wall and in landmine applications within range of Blackwatch arrows
- Chips of dragonglass get made into arrowheads
- Blackwatch will have the manpower they need to guard the Wall
As army approaches, simply ignite the mines. The undead only go forward, and they never turn around. Light 'em up and they burn to true death.
Archers pick out White Watchers and hit them with the dragonglass arrows. As Tarley demonstrated, even a mere scratch will blow them up into shards of dead white funk (truthfully he put that dragonglass dagger only about 6 inches into that Walker's back, which would hurt, but certainly not kill an even an ordinary man).
The Army of the Dead gets burnt up and can't come back - and the ratio of White Walker to Wight is so low that once the Black Watch picks off 20-30 there they'll turn around and head back up to their nothingness. Thoughts?