Yup, roll Nature if within, and half Nature outside if it must.
SG, p. 171
Monsters Acting Outside of Nature
A monster will not willingly act outside of its Nature descriptors.
If forced to act outside of its Nature by some quirk of the situation, use half of its Nature rating (rounded up) for rolls until the monster can get back on track.
Folk have fuller stat blocks in the Beasts with Two Legs section (SG, p. 201), so if you are rolling vs. an ability or skill, then you can test against that number if appropriate. Otherwise, if you are coming up with something on the fly and there isn’t a statblock, pick one that is close as a template.
As for advancement, enemies are always one level ahead of the character, so the general idea is that they are meant to present an ever-increasing challenge.
DH, p. 9
In fact, enemies are always one level higher than you.
They’re always raising the bar. How that really manifests is up to you. I’ve seen some GMs just have the NPC enemy of the same class have a rating one higher than the player (up to 6) to keep it simple.
I’ve seen some GMs write out whole character sheets for NPCs. This isn’t necessary for your first rodeo, but it helps some GMs to think about how to push the obstacle and increase challenges—especially if this enemy NPC is a common, reoccurring character.
NPCs don’t benefit from traits or instincts even though it can be helpful to know what motivates them. NPCs also don’t use rewards to Channel Nature or reroll dice, but I’ve even seen some GMs give Persona and Fate to NPCs to do that. However, that hack changes things considerably and should not be taken lightly. Conflicts will become more deadly and sometimes take longer, and the predictability of the dice pool becomes less stable when an NPC can suddenly double down. Some tables love it, though. Instead, I recommend giving NPCs helpers, gear, supplies, or magical/enchanted items to adjust the dice pools if you need to further juice the dice. Plus, this gives players agency to disarm weapons that are granting those bonuses.
Also, it should be added that enemies never make tests on their own unless in response to a player action. Most of the time, the stat number is for a vs. test. If they need to do something in the fiction, the NPCs should just do the action if it is setting up an obstacle, and then the players test against that.