When trying to build Magical Units, you'll want to look for passives and equipment that directly increase their INT and MP, or that offer distinctive advantages in casting offensive magic. You should learn a diverse arsenal of elemental spells, in order to effectively deal damage to all kinds of foes.

You'll also want to balance the amount of Unit souls you spend both on Awakening and increasing the SC cap. If you're just starting out, I recommend spending your souls on the first two Awakenings first, which will cost you 30 and 60 souls respectively.

This is because at first you won't have access to that many skills to equip your character with, so 20 SC should be plenty for getting the essentials. As a general rule, you should expand SC only when you need the extra capacity, just be wary that the soul cost increases every five levels.

Before trying to learn any of these skills, make sure your Unit can't already learn them via their Ability Tree. This is an easy way to waste a lot of Blue/Red Souls, since there's no benefit to learning the same skill twice.