Nus are interesting Monsters that generally appear in special situations. They have a lot of hit points, but they aren't very dangerous alone. Finding them is usually the real challenge.
The major exception are the six Nus in Enhasa. Three of them (in a triangle pattern) have one version of Head Butt, while the other three have the other version. Since one version sets your HP to 1, and the other deals 1 damage, this combination is lethal. Fortunately, once you take out all three of either type, the other three are much less dangerous. (In fact, the ones that reduce your HP to 1 won't be able to defeat you at all!)