Okay, so if we don't send a full war commitment through mass levy all were going to do is send out warriors to die against the Nomads.
If we do send out mass levy, we start taking hits to basically everything, but can throw enough bodies at the nomad to stop them from progressing further.
It seems like they are mostly taking advantage of the panic of the plague to use their mobility to ride down villages, since the cities, which would normally be immune to this tactic, are empty due to plague.
I doubt the Thunder Twins have any cities that survived, so we have to mass levy throw bodies at the Nomads so that they slow down enough to catch the plague and die horribly.
Unless it's the fact that bonding with horses makes one immune to the plague, in which case we are basically fucked either way.