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.

A casual google search uncovered several discussions about this, the most relevant apparently being this build. I am copying it here, with explanations.

Paizo BLASTER CASTER build:

Goal: Pile on the hurty-hurt with direct damage spells. You don't need battlefield control if the enemy is dead.

Level 1: Take Sorceror: Crossblooded Orc/Draconic, Human, take Varisian Tattoo, take Trait: magical Lineage (pick spell), Reactionary (+2 Init)

Then take Wizard/Evoker -Admixture Specialist for your remaining levels. Why Admixture? Because you can change the element of any of your blasting spells on the fly to get around elemental resistances/immunities.

If you want to superspecialize outside of Evoker, take Sin Magic, lose two schools (Conjuration/Abjuration), gain yet another spell slot per level of raw power.

Key Feats: Intensify Spell, Empower Spell, Quicken Spell, Spell Perfection, Spell Penetration, Spell Specialization, Greater Spell Specialization.

End result: Crossblooded sorc orc/dragon bloodline 1, Wiz/19, Admixture Evocation specialist. Note: Can use Sin Magic for more slots. Sorcerer level allows use of spell devices from denied schools (Conj/Abjuration). Magical Lineage Trait allows Intensify for free on chosen spell. Spell Perfection allows free Quicken at higher levels.

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Play Hints:

Take Burning Hands or Magic Missile as a Specialized Spell early. Burning Hands will deal more damage, Magic Missile has better long-term utility and keeps you out of danger. Every other level, you can change your specialized spell.

Add Greater Spell Specialization at level 7 or 9. Why? You can then memorize utility spells, and trash them for your blaster spell.

Change your specialized spell up to Scorching Ray or Fireball when you can, depending on campaign, typically at 5 or 7.

At level 12 or higher, change it to Fire Snake. Use Admixture specializing to change the element on the fly.

Use Fire Snake until higher levels. Why? High damage base and level 5 spell still leaves room for metamagic, esp Quicken.

Mechanics behind Choices: Orc blooded, Draconic: +1 to all damage spells, +1 to element of choice, retasked by Admixture = +2 dmg/die on blasting spells.

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Feats

Intensify Spell: Increases caster level damage cap +5 to apply to a specific spell. Burning hands goes up to 10d4+20. Magic Missile goes to 7d4+14. Fireball to 15d6+30, Fire Snake to 20d6+40.

Empower Spell: Increase dmg by 50%.

Quicken Spell: Hit enemy with two spells/rd.

Spell Specialization: +2 to caster level with a specific spell. Helps bring the damage earlier and faster.

Varisian Tattoo: +1 to Caster level with a specific school (Evo). This buys off your sorc level.

Greater Spell Specialization: Sacrifice spells to power your chosen blaster spell. Means you can memorize utility spells freely.

Spell Perfection: Doubles fixed feat bonuses, apply one metamagic for free. An Empowered/Intensified spell with Magical Lineage is still its original spell slot. SPell Penetration doubles to +4. Varisian Tattoo to +2. Spell Specialization to +4. Effectively, you've got +10 on Spell Resistance rolls, and are casting 'Your spell' at 5 levels higher then your own.

Top End Damage: 30d6 +60 from Fire Snake, empowered, Intensified, average 165 dmg, save 1/2, level 5 slot. Quicken for another hit, 5th level slot, 20d6 + 40dmg, avg 165. = 330 blasting dmg in one round, save for 1/2. If you've a Rod of Maximize, you can lift this to 215 base damage.

Base level 5 spell slot is 20d6+40 dmg, 165 dmg.

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By Level

At level 1, your Burning Hands should be 2d4+4 (avg 9)

At level 2, its unchanged.

At level 3, Spell Specialization kicks in. 5d4+10 (22.5). This tops it unless you Intensify it.

At 4th, 6d4+12 (27), Intensified BH.

At 5th, Intensified BH, 7d4+14 (31.5).

At 6th, you can shift Spec to Scorching Ray. 2x 4d6+8 (44). Your Fireball is 5d6+10 as well, or 7d6+14 if specced.

At 7th, 6d6+12 fireball, or 8d6+18 if spec. An Empowered, Intensified Burning Hands, if still the spec spell, is 13.5d4+27 (about 60).

8th - Empowered Scorching Ray, 2 x 6d6+12. E/I BH is 15d6+30 (74, max)

9th - Intensified, Specialized Fireball is 11d6+22 (51).

10th - Firesnake. E/Spec Fireball is 15d6+30 (74). Emp Scorching Rays are 3 x 6d6+12, or 18d6+36 (99 dmg). You can now Quicken a Burning Hands or Magic Missile as kicker damage in a round, although you've few slots.

12th - E/I/Spec Fireball is 21d6+42 (115) damage. You can now Quicken a 12d6+24 Scorching Ray.

14th level - A Specialized Firesnake now exceeds/equals an intensified Fireball. Fireball caps at 22.5d6+ 45 (123~) damage. An Empowered Firesnake is 24d6+48 (132) damage. You can now Quicken a 10d6+20 Fireball. Intensified, Empowered Scorching Ray tops out at 24d6+48 (132 dmg)

15th level - Spell Perfection. You can now add a Meta for Free. This will be Quicken or Empower. Intensified might be free if Magical Lineage applies to it. Caster level buffs for spec spell exceed +5, so top out at 20d6 dmg at 15th level. You will miss Spell Resistance rolls against CR appropriate enemies on a 1. Assuming Firesnake, you can now cast a Quickened Intensified Firesnake for 20d6+40 (110) damage out of a 5th level slot, and an Empowered Firesnake out of the same slot for 30d6+60 (165) damage. Using a 7th level slot, you can Empower both.

16th+ - Damage remains the same, higher level spell slots are open for use of other Metas or control spells.

Conceivably you could use Disintegrate to get a higher damage total, but the delay isn't worth it, and you'd lose the Varisian tattoo bonus.

If your DM allows you Twin Spell from 3.5, you can very, very easily clock in at 495 raw dmg/round. IF he allows Arcane Thesis, god help your enemies.