Once you have Combat V/Shock II, it's very rare that you'll lose the unit if you're reasonably careful. Also, Valor + GC = minimum 3xp per battle, and level 15 is I believe 197 xp. Charismatic probably lowers that to around 180 xp. If I've got my math right, level 12 is only 122 xp, which is FAR too low for getting a unit as powerful as Brigit is. Again, she's an immortal that you can get far in advance of the tech requirement.
As for which units usually make it to level 15, if I'm not specifically going for a level 15 unit (like I was in my Malakim game since I spotted the Ring early):
Valin Phanuel/Rosier the Fallen - Quiet easy. With Mobility II and Commando they can easily and safely pick off enemies from very far away, and high withdrawal means you can risk them on lower-% battles. Blitz helps too.
Alazkan - Very very easy. HN = free XP-farming.
Vampires - Usually don't bother feeding them that high unless they're archmages, but very easily possible.
Adventurers - Easy if you want to do it, since you can start very early.
Immortals - Easy, but come extremely late-game. Just keep throwing them at the nearest enemy

Occasionally I'll have a Flanking III horseman/chariot that lasts until level 15, but it's not that common.
One way to get high-level units is to go hero-hunting. Remember, world units are worth 4 times the XP, so a single kill at low odds can be worth a ton of XP.
Finally, as for the game being over by the time you get a level 15 unit, it depends entirely on what unit you're training up. Immortals, Vampires, and Alazkan are all reasonably late-game, so yes chances are you've already won. Lucky warriors/axemen can come very early, as in my case. Rosier and Valin are fairly early too.