You can't learn everything through the Master/Student relationship, only generic magic spells (fire, wind, ice, star, heal, espoir and their upgrades) and buffs/debuffs (again, only the generic ones from healers and mages/skulls).

As for the spells that you can learn, you need to keep casting them until they get to lv 1+ (they start at lv 0, 4~5 casts is usually enough to reach lv1)

Enemy units in the Item World have pretty much everything randomized (at least to some extent), including what skills they have.

Darn, so I won't be able to have an Undead with Stone Breath, Hell Pollen, and Tie Down.

Originally posted by viirinsoftworks:

Darn, so I won't be able to have an Undead with Stone Breath, Hell Pollen, and Tie Down.

I don't remember if it works, but try learning those skills and then reincarnating as another class. maybe that way it works. or maybe not...

Originally posted by Zeon08:

I don't remember if it works, but try learning those skills and then reincarnating as another class. maybe that way it works. or maybe not...

Nah, that doesn't work. But your comment did remind me of a bug in the game - at least one that was present in DHD, I never tried it myself so I can only assume it works in this version as well. Basically, you can do a magichange 2 on the map with Vyers and, after the secret ending, if you follow certain steps, you can keep the magichanged weapon(s) and any abilities the humanoid unit(s) may have learnt. Again, this is a bug, so use it at your own risk and discretion.

Here's the video where it's shown and explained in details:
https://www.youtube.com/watch?v=LQHkk4H08Ao

Looks like Marona (Phantom Brave) is the one where all abilities can be learned by everyone depending on what you do, legitimately. I wonder if Disgaea 6 does it too?