The AI factions are supposedly biased to attacking those factions/territories that they historically fought, so it should be quite likely that Huns will end up fighting the WE.

As for the withdrawal starategy, I would suggest using scorched earth tactics with it. Leave a peasant unit (or what ever is the cheapest unit type) in each provincial capital so that the barbarians have to lay siege for at least 1 turn when they attack it. On that turn, destroy all infrastructure so that they will have to rebuild from scratch. That should slow down their expansionism quite a bit.