13th level Humanoid that's able to evade a 18th level party? That's a decent CR gulf to span. Classes alone probably won't do it, since classing something to level 13 means it should be balanced against other 13th level creatures, though min/maxing with an emphasis on evasion might slant that.
As a magic item suggestion, give the NPC a unique Artifact of your own creation that gives them additional evasive powers; probably something that locks to their person so high level PCs can't just snatch it from them to make them powerless. If you want them to just be hard to nail down, have it grant etherealness or earth glide (if underground a lot). If you don't want the Artifact to be a loot item the PCs get afterward, you might try something from the Weapons of Legacy supplement, where the item in question is an item that has quests/requirements to use it to its full potential, and the NPC has done the quest(s) but the PCs haven't/can't.
For the Lycanthrope aspect, normally a Lycanthrope build has to use an "animal" as the base creature type. Can you have this NPC through some means be a non-animal lycan with additional abilities? Create a Lyncanthrope build for a Displacer Beast or similar hard-to-nail-down beast; then the NPCs evasive abilities are intrinsic.
Empowering a prophecy without railroading, using mechanics, is very much possible. The "stick" is not so useful here as is luring the player with the "carrot". There are a number of more-or-less successful games that do this with great success*, to the point that some use it as a central part of character development, so there's evidence in the wild that this is possible.
Substantial bribes
All the games I've observed doing this effectively offer a significant "bribe" that tempts players to choose to act in line with their fate. Such a bribe must be large enough to give the player a true dilemma – do I act with freedom to choose the "best" course of action for my character, or do I take the big mechanical boost and do what the prophecy says I must?
The bribe has to be big, unbalancing almost, in order to make it large enough to be the equal to how much a player values their freedom of choice. The extra that they're getting must be valuable enough to really make them consider it. In D&D terms, a +5 or more to all skill and hit rolls for the duration of the time where they are facing the direct consequences of their choice (the resulting battle, the escape from the crypt, etc.) might be suitable. Another effective carrot is the game's improvement currency: a large XP bonus (perhaps 10%, or more, of what they need to level) can be very tempting for a player.
This can be tweaked as you go, to. You don't have to tell them at the outset that every such moment is worth 1000 XP — instead, when they come to a pivotal moment in their fate, you can mark it as such by saying "…and if you choose X against your better judgement, it's worth N experience." You can offer variable carrots tailored to the importance of the choice, too, so that you give bigger bribes to properly balance the sides of the dilemma.
Free choice
It must always be a free choice. You might set the dilemma, but a player hugely values their freedom to choose.
By putting the choice into your player's hands, you make it more likely that they will fulfill parts of the prophecy/curse/fate and you don't railroad it. As long as you're always prepared to accept the player's true choice (don't up the bribe after they refuse, for example), they will feel it really is their free choice, and you'll get much more buy-in for when they do choose to accept the prophecy.
Not everyone likes talking mechanics
The one caveat is that some players who go for deep immersion will really dislike baldly discussing mechanics when they're trying to choose according to their character. There's no real good way around this without seriously annoying the player and undermining their buy-in. The compromise application of this idea is to not tell them that they have the choice – but when they do choose the path of prophecy, narrate how their sword swings truer (as you start appling a hit bonus to their rolls) or how a great weight of foreboding settles in their chest (as you give them bonus XP as an aside). This will create the association between bonuses and accepting their fate, without interrupting their internal narration and breaking their suspension of disbelief.
* One example is The Riddle of Steel. When you're acting in line with your Fate during a pivotal moment in the prophecy, you get a very large bonus to all your rolls during the fallout of the choice. Another is Dungeon World, where you can get into situations where if you do a thing that you normally wouldn't choose, you get to mark XP.
Best Answer
One option might be to turn to real-life fortune-teller-esque techniques. Essentially, psychics use these to demonstrate their great insight to the audience. While these could be useful tools for convincing others that you can see the future, you might also use them as they were originally intended for some flavor. I've included the Wikipedia links, because they do an excellent job of explaining and expanding upon them.
Cold reading
Cold reading is tricking your audience into thinking you have powers based on general psychological knowledge. This boils down to making broad statements, likely about a large group of people/things, such that the statement will be true for at least one of them. For example, your PC might say, "I sense impending misfortune. One of you will fall sick in a short time," to a group of people. In almost all cases, this will be correct. Alternately, you can predict something common, but notable, about a single person's future. You might tell a shopkeeper that they will have a fight with someone in their family, or tell an adventurer that they will be injured. In general, keep your statements vague,
Hot reading
Hot reading refers to using prior knowledge, gained without the audience's knowledge, to pretend that you have special insight. This might be more mechanical; you might secretly make Listen checks to overhear conversations, Sense Motive checks to determine truth from lies, or Gather Information checks to find out more about someone, or, of course, just read their mind via magic. Then, you can present it as if you used psychic powers to determine it. You might use these skills to determine information, then interpret it yourself, or ask your DM to let you roll to use your character's Wisdom to interpret it.
For example, you might ask around about the Lord of Greenwood, find out that he loves to fight, and that he will hold a tournament next month. Then, approach him, presenting yourself as a stranger with no knowledge of the area, and "sense" that he'll soon become greatly admired for his skill. This statement stays vague (after all, you'd be in trouble if you predicted that he'd win and he lost in the first round!) but is still specific to him, and he will take it positively.
To read someone's personality, make vague, positive statements about them. These are commonly known as Barnum Statements, and is sometimes called warm reading. If you make these statements contradictory, in what's known as the rainbow ruse, they will always apply. For example, if you say, "You are a warm and kind person, but sometimes you're unable to show how you feel," it will appear that you have deep insight about them, but the statement applies to just about everyone.
Overall, remember to keep your statements vague and open-ended, and allow the people you tell the fortunes of to interpret your words themselves. If you want to keep the fact that you're not really a fortune-teller secret from your fellow players, pass notes with the DM, even if they're actually blank. That way, it looks like you're using your powers, when you're really just using simple ways to make statements apply anywhere.