Sage

Sage is an utility class that amplifies other spells you already know. Thus, it is best to only take sage if you already have some spells from other classes.

Increased Reach is a situational, but useful tool to make those spells that have a low range easier to use.

With selective casting, AoE abilities that can also hit allies before a lot easier to use. Even something like a meteor is now perfectly safe to stand in for your allies.

Lengthened Incantation can be used to increase the damage your spells deal. It is most effective when used with AoE attacks, as the damage bonus would be effective against all enemies hit. You can also use it to gather damage for 1 big attack when dealing with high guard enemies.

Dualcast effectively allows you to give a spell the rapid keyword for a turn, though with some more restrictions. There are many potential ways to use it. The low mana cost makes it easy to use it even for things like tossing two fireballs. Lengthened Incantation can provide a similar amount of damage, but dualcast has the advantage of being able to see the results of your action before you commit to casting it again.

Secret Art: Overcharge can be seen as a free 3 mana once per encounter, at the cost of using your secret art. It does require a high mana spell to get maximum use out of, but unless you have another secret art it competes with, it's worth using every fight.