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.

As per Adeptus's answer, officially and without optional rules, initial training isn't quantified. Further, while level 0 creatures were a thing in previous editions of Dungeons and Dragons, they usually aren't in Dungeons and Dragons, Third Edition and many of its offspring. A creature without racial Hit Dice has a class level, even of commoner.

Below are two options you can use to determine a character's initial training time.

The hard numbers way from the Dungeon Master's Guide

The following times and costs are gleaned from the Dungeon Master's Guide using the optional rules on How PCs Improve (197–8). These rules are deliberately vague, the DMG expecting the DM to compose his own house rules for such training sequences, but I've summarized the DMG's suggested times and costs on the chart below.

  ADVANCEMENT     |       TIME           |              COST               |
------------------|----------------------|---------------------------------|
Skill Rank [1]    | 1 week/rank          |                      50 gp/week | 
------------------|----------------------|---------------------------------|
Feat [2]          | 2 weeks              |                     100 gp      |
------------------|----------------------|---------------------------------|
Spell             |                      |                                 |
 Wiz/Brd          | 1 day/spell          |  (spellcasting cost ×2) gp [3]  |
 Sor [4]          | varies               |     appropriate service         |
------------------|----------------------|---------------------------------|
Class Benefits    |                      |                                 |
 with trainer [5] | 1 week/2 levels [6]  |                   1,000 gp/week |
 w/o trainer      | 2 weeks/2 levels [6] |                   1,000 gp/week |
------------------|----------------------|---------------------------------|
NOTES
[1] A character can simultaneously train in 2 skills, paying full price for each.
[2] A character can simultaneously train for 2 feats, paying full price for each.
[3] Spellcasting cost is as per Table 7–8: Goods and Services under
    Spellcasting and Services (PH 129).
[4] To gain spell knowledge, sorcerers seek out magical creatures and
    perform services for them. No, really.
[5] Trainer must be higher level in the class in which the character seeks
    training.
[6] Rounded up (for example, from nothing to Bbn1 requires 1 week, from Brd2
    to Brd3 requires 2 weeks).

Thus, for example, for a human blank slate with no previous class or feats and an Intelligence score of 10 to become a level 1 rogue spends 36 weeks and 1,800 gp to learn his 36 skill ranks, 4 weeks and 200 gp to learn his 2 feats, and with a trainer 1 week and 1,000 gp to gain the class features of a level 1 rogue. The human's finished his training in 41 weeks and paid an awesome 3,000 gp. The time (but not expense) could be reduced by training skills and feats simultaneously.

These rules cause the universe to pump out clerics with ease (low skill points and no spell acquisition) and make wizards extremely rare given the relatively enormous price of acquiring their spells.

These rules also mean creatures with low Intelligence scores complete their training before creatures with high Intelligence scores have completed theirs.

Which is fair enough, I suppose.

The make stuff up way from the Player's Handbook 2

First, a creature goes on a rebuild quest as described in the Player's Handbook II. These are designed for rebuilding existing PCs, so the language below is specific to those creatures.

A quest to rebuild a character should excite and frighten players. It’s an adventure few characters would willingly embark upon and fewer still could survive. To make rebuilding an option at all levels of play, however, the degree of challenge must change according to the PCs’ level.

[An objective the DM determines] represents the final encounter before a character can undergo rebuilding. The PCs might need to embark on a perilous overland journey or a long dungeon crawl to get to this encounter.... Set up the preliminary adventures in whatever way best fits your game and your players’ desires. (199)

However, nothing prevents the DM from saying that a creature, for example a human commoner 1, underwent a rebuild quest before the game began. (Perhaps this is the character's origin story.)

Then, once the rebuilding quest is complete,

[Y]ou can change a number of levels equal to 1/5 his character level (rounded up) from one class to any other class (or classes). (198)

Thus completing that rebuild quest could, for example, replace that human commoner 1's level of commoner with a level of wizard.

As the quest can be of any length, this makes the so-called training time (actually, adventuring time) for acquiring a creature's first level as long as the DM wants it to be.