Can't use sacrifice in battle with giant spider?

[Metathron]

I'm playing as a mage on hard, I cast hypnotize on a stack of some 120 venomous spiders, and in the next round I wanted to sacrifice them to resurrect my badly depleted stack of sprites - the dang spider targeted them a couple of times instead of summoning more spiders!

So, what gives? Is there some kind of ban against using certain spells in a battle against bosses? Even though I can use it on my own units!