Ask a Magician Thread.

MP eater would be good if monsters didn't have such low MP. I wouldn't bother maxing it; it's more useful for clerics.
As for Cold Beam, I used it well into 3rd because of it's range, and I still use Medi. (It stacks with maple warrior.) As for slow, I did not max it because I/Ls aready have good mob control, but I kinda wish I did. Some bosses (Pap, Giest) can be slowed, along with other monsters that cannot freeze. But why would you train on those anyway?

Does Cold Beam have more range than Composition or Thunder Spear? I'm narrowing it down to Eater and Cold Beam, I think, unless a low level Slow is useful at bosses.