How to deal with elephants

[Lazy_Perfectionist]

In army like Ulm, with low defense units, you might consider a spell like Will o' the Wisp. I've never used this spell yet, but it strikes me that they might actually be useful. High defense, ethereal, fire burst attack, and three hitpoints- enough to withstand three failed tramples.

Since I've never used it, something I'll remedy ASAP, can anyone tell me how the spell works? If you cast it the first turn, will they arrive in time to intercept the elephants?