Maybe the death field stops at dead material. So, sand, clay, and such would shield them.
I doubt Wendy will be much hurt by whatever the nasty pedo has in mind, but unfortunately I could see him kidnapping her for his organization's nefarious purposes. Likely those purposes require her rare magic, I can't see any other reason for targeting her.
At the moment I don't think the death mage has anything to do with Mest unless he's Mest's past victim... Still, having two separate arcs running side by side seems unlikely in a series as unimaginative as Fairy Tail, so they must have something to do with each other unless the death mage is just going to reveal something that will place the foundation for a later arc.
It would have been pretty funny if Mashima had first built a careful plot around whatever evil Mest has in mind for poor Wendy, let it (and all the audience's rage) mature for a number of chapters, and then got Mest completely accidentally killed by this death mage. That's what I'd have done!