Use those scrolls

[vdx]

Use incantations and tomes (scrolls) to re-buff rather than re-casting the individual buffs. Doing so will save AP and SP in a fight as well as preserve buffs from other scrolls.

For example, a  Lv. 27 current party uses five enchantments regularly, for a total costs of 14 AP and 176 SP. Casting before a fight is just a matter of a few clicks but trying to re-cast in the middle of a fight can be awkward given the need to use AP to attack and the limited range, AoE of buffs. In contrast the use of a tomb is simply 1 AP from any character with no limits on range or spacing (map-wide AoE). Note: Enchantment casting costs vary depending on the number, type and level of the buffs.

Added plus, scrolls of sustaiment also increase the duration of buffs from other other scrolls, such as as the Incantation of Ice (+30 ice damage for 3-5 turns), or the Incantation of the Furnace-Maw (+70 fire damage for 3-5 turns). Thus, a party that uses scrolls of sustainment can turn a limited turn buff from a scroll into a boss fight long buff. Or if you are really flush in gold, buy a ton of sustainment scrolls and you can keep the +70 fire damage from Furnance-Maw through an entire quest (camping and using an inn negates all buffs so you'll have to heal via potions).

Scrolls are found as loot and can be purchased from item shops: 
Purchasing price (V2.2.41):

Incantation of Sustainment 280g (Burnehorn etc)

Tome of Sustainment: 1100g (Gholla Outlook, etc)
(Update: As of v3.1.1, Nov. 2014, Tomes of Sustainment retail at 1800g).

AP Cost to use: 1 AP (like all items).

I went to Gholla Outlook before entering the Cult Temple. Having a few tomes of sustainment was very helpful in fighting Big V.

[slayernz]

By the time you get to Gholla Outlook, you should have a good amount of gold. If you do, I STRONGLY recommend you buy up a fair number of the Tomes of Sustainment. I bought 10 before I went and did the Cult Temple, and it made a world of difference. In fact, it was that much of an aid that I made the trek back to Gholla Outlook and went and purchased an additional 50!
I'm one of these players who tries his hardest to avoid using potions, but when you get sustained fights, they are the thing that keeps you from being dead.

[fallen]

Especially in the Underforge, I can heartily agree - Sustainment Scrolls and Tomes are critical.

[mrsarfa]

When using Fyona as either your group's main tank or off-tank, I've also found sustainment scrolls to be useful to extend Fyona's Consecrated Defense during long boss fights with waves of enemies. She may not always be able to get the first hit on an enemy or may be busy healing other party members and not have a chance to attack, so having the buff last for many rounds is a great way to keep her defense up.