Slightly-high CR can be deadly, even on full-resources.
When putting together an encounter or adventure, especially at lower levels, exercise caution when using monsters whose challenge rating is higher than the party's average level. Such a creature might deal enough damage with a single action to take out adventurers of a lower level....
In addition, some monsters might have features that are difficult or impossible for lower-level characters to overcome. For example, a Rakshasa has challenge rating of 13 and is immune to spells of 6th level and lower. SPellcasters of 12th level or lower have no spells higher than 6th level.... Such an encounter would be significantly tougher for the party than the monster's challenge rating might suggest.
"Challenge Rating", DMG5e p.82, emphasis mine.
Yes, many parties will be able to punch "above their weight" if they're going into something stock-full of spell slots, 'once-per' class features, &c. But I think the advice above is well-heeded in your case. Level 1 characters, in particular, can drop rather fast.
(Just the other week I had a first-level party walk into an ambush. The wizard was first in marching order (!) and a crit from a sling (!!) dropped him during surprise. It only got worse from there. Certainly those players didn't manage it well, but there's no reason your werewolf might not go for the "sparkle-guy" first.)
So how do you gauge the difficulty? Game it out yourself! Grab four prototypical characters and throw them in a room with a werewolf. See how they do. Sure, the action economy's on their side. But the werewolf's going to have resistance to mundane (non-silvered) damage. AngryGM's got anarticle on combat encounters which nicely demonstrates this mini-playtest approach.
[Warning: AngryGM's posts feature vulgar language, though likely no worse than you'll hear on basic cable. Certainly worse than you'll hear here, though.]
But objectives matter...
Is your boss cornered? Is he so single-minded that he'll fight to the death when escape is an option? Why is your boss opposed to the characters? Why are the characters after the boss? These are crucial questions in the encounter's construction. You could throw a CR6 Kuo-toa Archpriest in their way who won't be deadly if...
- He's keenly afraid of any attention being drawn to his presence
- He'll blanch at the sight of any blood
- He's got plans for the party and wants them alive
- He's got plans for the party and just wants to draw them along
- He's in his prepared defenses and just needs to flip a lever to keep the party at bay
- &c. &c. &c.
Your werewolf probably doesn't want to die any more than your players' characters do. Use that.