The spell Body ethereal

[Lex]

having the spell only affect an adjacent square is not so bad. If you order your fighters to hold a turn, any flyers attacking your rear will find your casters embeded in your army and will be cut down in short order. Also definitly leave buffer troops up front to attract enemy fire. Staff of Storms is a must, and missile protection, if you can afford to mass produce the items, could let you try more daring tactics. Once your troops are buffed, your casters can always fall back on Stellar Cascades to prevent enemy cavalry from rushing past your army and taking out your casters.

But I've found each path can be really powerful when you group large numbers of casters in combat. Astral is no exception.