For example, Purple Sun is a terrible defensive weapon. It's indiscriminate, its area is too large, and there's no way to defend your own troops from it. It's a terrifying offensive spell. Outfit a high-performance gyrocopter with stealth gear, then replace its standard payload with an item that casts Purple Sun. Just buzz around the enemy army or fortification eradicating bits of their civilization. It's the equivalent of a nuclear bomber, except nobody's yet invented the ICBM, radar, surface-to-air missiles, or even interceptors.
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Just stack up every reroll/precision buff into a single item, then dump the wombo-combo on high-value low-probability precision actions like "Mount a runed ballista on top of Karag Lhune and snipe the orc warboss from across the caldera". Premonition, First Portent of Amul, Omen, Lens on the Sky, Eyes of Truth, Enchant Item, Trial and Error, Law of Logic, etc. I think that's three rerolls, two "will trying it now be a good or bad idea", two supernatural performance bonuses...