DEAFLOPIST'S PROFILE

The purple-cloaked person in row 5, column 4 looks like the kind of character that nobody'd like to pick for fear of being called edgy. So, naturally, that's my choice.

At the start of a battle, you could run an event at turn 0 that checks the current time and adds/takes away stats accordingly through changing the enemy. The stat change could be done by transforming the enemy into an identical looking one with the altered stats at the start of the battle (turn 0 event in troops). An example is having the version of the werewolf appear with 500 health initially, but if the battle starts while it's daytime, an event in turn 0 transforms the werewolf into a enemy that looks and is named exactly the same but with 250 health, essentially meaning that it has twice as much health at night. It'd be a seamless transition and the player wouldn't notice(unless you want them to, in which case you can add some fancy animation/text in the turn 0 event), except that the werewolf would be significantly easier to kill.

I'm not sure if the time changes in the middle of a battle, but if it does, you can add a "when end of turn & turn 1 + 1*X" event as well that also checks the time and transforms the enemy accordingly. This wouldn't halve the werewolf's health, however, as it will just change a 500 health wolf that is damaged to 400/500 health to have a maximum health of 250, which won't half it but damage it to that point.

This is the solution that I've used for similar ideas, i'm sure there's cleaner and more flexible methods as well.