NerVNotDeaD
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I remember defending against an Academy's castle siege; I was playing as a Dwarf. I used Fireball to destroy their ammunition, and when only 200 defenders remained against 6 Titans, the Endless Lightning spell helped finish them off. However, in a field battle, Thunderstorm might be more useful – it could force enemy archers to spend a turn running away. Otherwise, it's just an auxiliary unit (and a very expensive one at that). I often don't even upgrade it; Rakshasa and Golems are what really matter.