Armies didn't decide 'we need heavy infantry' and design their organisation around that - technology and existent tactics demonstrated a need for them and they emerged. If you are thinking 'need xyz military units like in an RTS' you're going about it backwards.
Instead, think about what level of technology (this includes magic) and equipment they have available, and how this would shape tactics. Think up an army organisation, then defeat it with a more 'suitable' one using the same equipment and tactics/commuication/etc. Continue doing this in thought experiments until it gets hard. Don't go in with pre-conceptions; let the tactics for a given set of abilities emerge naturally.