I think that shooting for Conj 3 ASAP is a good plan as Agartha. 5 Magma Children for 2 gems (!!!) is nothing to scoff at and the Earth Elementals are great arrow pincushions and tramplers.
The Magma Children are really, really easy to mass since every four of the Earth Readers can cast it. They work well in synerge with for example a Red Dragon pretender. The MG's are kind of vulnerable to ranged attacks, especially javelins, but their attack does nasty damage on ordinary, and even a bit more extraordinary units.