Can someone explain level draining?

[Kaytara]

Can he just permanently lose SOME of them? I mean, that would be kind of brutal since they take so long to earn! He must be virtually useless with his level dropped so much.

From my limited understanding, it's not QUITE that bad.

...As long as it's mostly just Elan.

What matters is the kind of critters the party is still able to tackle despite Elan being level-drained. If the party is currently level 15-ish, as they seem to be, and aren't too badly impacted by Elan being more useless than usual (okay, to be fair, he's been getting better), then Elan will get a bigger chunk of XP with each party kill than he did back when he was originally earning those levels (when his party's average level and the monsters' general CR was lower), so he'll be earning them back quicker than he did the original ones. It wouldn't take THAT long for him to catch up with the others again.

...Not that that's much of a consolation.