Many of your Marshmasters get Astral, and you have unlimited training capacity for Shamans as communion slaves. An 8 slave communion will have Marshmasters casting at 4W/4S/5D/5N, before booster items. This gives you a wide array of BE to cast, as well as the potential for devastating, sustained artillery barrages. I took over MA C'tis in a game recently, and after exploring them more..... Well I think they're a good target for a full playthrough, and maybe write my first guide.
The biggest point to remember with C'tis though, is the Marshmasters. That unit ties the entire nation together. The only reason to not train a Marshmaster, is to train a Shaman for research/slave. Basic rule of thumb would be to train a Shaman every time you get a Marshmaster with an Astral pick. I find keeping a balance of masters/slaves to be an effective way of keeping your fatigue in check over the course of a big battle.
Skellie spam can be very effective, but from my observation, it is most powerful early on, and slowly wanes in usefulness later. Though putting 3D Marshmasters in the army to just bulk your line with longdead while the communion does the real work, is viable.