This never ever happens. If an alliance wants to take your alliance out, they will send ALL of their units at you. Regardless of chasis type.
^^ Well... maybe I'll find this out, soon. No one's attacked us, yet so I honestly have no idea what to expect.
@Ferr3t: That makes a lot of sense... although it would seem that in either of those scenarios, the Heavies will be completely wiped out by those Cavalry. I get why the Mixed army would get wiped out, however - Slash, Pierce, and Crush Cavalry units would have an affinity bonus against all opposing unit types.
I simulated a battle using every mixed unit type up against the equivalent of a focused Cavalry force. Needless to say, the Cavalry forces still had 2/3 of their ranged forces left standing after the fracas.
I assume the way one would have to engage in battle with that would be the utilization of allies, missiles, and spy attacks, then? In some kind of combo.