Every enemy profile carries a health figure and a level curve running to 50. Both are more useful once you know that the figure is per segment, and that segments are not evenly handed out.
Segments multiply the number you read
Of the 84 enemy profiles, 49 have a single health segment, so the printed figure is the whole fight. Twenty-one have three. Six have five. Lacra and both of Mihai's forms have nine, which is the highest in the game.
Brencis carries 45,716 health per segment at level 50 and has three segments, so the knyaz is a 137,148 health fight at the cap. Ambrus has one segment and 65,308, which is a smaller total in a fight with no breathing room between phases.
Day and night are the same fight for all but one
It is tempting to assume enemies are softer at night, when you are strongest. They are not. Of the 84 profiles, 83 carry identical day and night curves at every level.
So the night advantage is entirely yours. It comes from your form, your vampiric abilities and your traversal, not from any weakness the valley develops after dark. That is worth knowing before you delay a fight for a bonus that does not exist.
The single exception is a night creature
One profile does differ, and only because it has no daytime figures at all. The Castaway exists at night and not during the day, so its night numbers stand alone rather than beating a daytime version of itself.
Heavies and rogues are mirror images
This is the most actionable line in the whole enemy set. Heavy variants, whether bandit, human or soldier, reduce incoming physical damage by 10% and take 10% more magical damage. Rogue variants do exactly the reverse.
Read that as an instruction. Bring Witchcraft to a heavy and steel to a rogue, and you swing a fifth of the damage in the exchange without touching your build. Uriash sit apart: they reduce physical damage by 10% and carry no magical weakness to trade for it.
The curve is steeper than the level number suggests
Ambrus goes from 1,000 health at level 1 to 65,308 at level 50, which is a factor of 65 across the run. Levels are not a gentle slope here, and a fight four or five levels above you is a genuinely different fight rather than a longer one.