The Night King throws a big spear at Viserion, killing him.
Taking in account how big the spear is, how far he threw it and...
how difficult is to pierce dragon skin
...we deduce that a ballista would be needed to achieve such a throw.
That means that the Night King (or White Walkers in general) must be incredibly strong.
However in some episodes we see Jon Snow sword fighting white walkers and blocking their sword attacks.
If they are so incredibly strong, wouldn't be impossible to block a sword attack?