Wyverns are fantastic and I personally like Vine Ogres. 1 Nature Gem for 2-3 Ogres per cast (or more depending on your equipment) that are enough of a threat that they have to be targeted because of their high HP and decent damage; but not expensive gem wise.

I have never had a Tartarian Giant die on me yet because he was decaying; they just need some blush. They don't radiate disease AFAIK.