Hmm, I've always had problems with my mages and guards in the rear being attacked by fliers. At some point I placed a decoy squad behind the mages which seemed to help.
I've now tested this with three squads of Caelum infantry, two set guard commander with a commander on (hold)(hold)(attack rear) and one set to (hold and attack rear). One archer squad was also there.
Defender were four squads of Marverni infantry and one druid with guards at the very rear.
Result:
The behindmost squad was those that was set to (hold and attack rear).