Mandragoras can kill almost any SC but they are undead and have low HP . So a few Priests or mages will decimate them extremely quickly . Imo not worth the nature gems .
Lamias are not bad but i would rather invest my always scarce nature gems in Ivy kings , Jade armors , GoH , GoR and maybe Haunted forest .
Summoning a bunch of Ivy kings and equipping them with Penetration bonus items for Charm spamming can be pretty effective if you can combine it with either a devastation battlefield spell like Wrathful skies or with Master enslave to take out the mass of troops
. Then with 5-10 Ivy kings spamming charm you have good chances to steal lots of enemy thugs/SCs .
The lamiaqueens themselves are excellent mages also .
So if you can afford don't summon nature creatures . If you want creatures look in earth and bloodmagic instead . Mech men , Devils , FoDs and living Statues are the classics here
.