Is soul drain the best combat spell in the game?

[Slen]

Alneyan said:
It's only 300 fatigue for an Air 3 mage; it's 150 fatigue at Air 4, 100 fatigue at Air 5, 75 fatigue at Air 6... You can get Summon Storm Power for a cheap +1, and add extra gems to lower the fatigue amount: Storm Power+two extra gems result in a level 3 mage being able to cast the spell with only 75 fatigue. On an aside, fatigue is capped at 200, so you can cast a 700 fatigue spell without doing anything special.

Relief is another great spell for fatigue, as it removes the fatigue of *all* your troops on the battlefield. If memory serves, Relief is at level 7 of research, and requires Nature 3... probably in the Enchantment school.

*Squashes the Figure Fairy out of this post*

.....how do you use EXTRA germs in combat to improve spells?