[Kobal2]

Any single target damage spell will target the Behemoths first (because of high HP and size) so early on Thunder Strike, Flame Bolts can do the trick. Later on you can count on Petrify, Disintegrate, Incinerate... Also note the Behemoths aren't Mindless like most undead (presumably because the necromancer mahout isn't) so Paralyse, Mind Hunt and Soul Slay all work, as do Charm and Enslave.

Of course, there's also the even cheaper option of massing archers or crossbows - Behemoths only have 7 protection so unless they're ethereal and you can't cast Flaming Arrows, missile fire of any kind will schwack them in short order (or cripple/limp them which is the next best thing).

I wouldn't really count on Earth Meld, Vine Arrow/Storm of Thorns or Fire Cage/Prison of Fire though: Behemoths are high strength and ridiculously high morale so at best those spells would only hold them for a turn... assuming the mage even targets them - AoE spells tend to target large concentrations first and that often means the skellispam screen. Besides if they're ethereal, one turn won't be enough for your guys to dispatch them unless you've also got a fire bless.
Crushers would be a good albeit expensive option to hold them in place however. Size 6 means they won't get flattened; mindless so the fear doesn't matter; and they've got better stats than the behemoths so they should come out on top of a trunk vs. fist contest even with minimal buffing or magic support.