Don't counterstance, having a PDT set will drastically help your mages but isn't required, BRD should sub BLM for elegy. Biggest difference vs a chariot fight is that you need to get the positioning perfect, though that's still not that much worse than ZR. As stated earlier, MNKs need to be in a V formation so that mega scissors only hits one of them and mages can stand within casting range of them while not being in range of venom shower.
http://img339.imageshack.us/img339/26/68270488.jpg
You don't need height differences to avoid any of the T3 moves on back line unless your mages are too stupid to understand distances, Krab is no exception.