[Zurai]

There are lots of spells that protect your troops from missile weapons, though some of them are slightly high level. Storm (air 4, evocation 5) calls a storm that ruins bows, Mist (air 3, evocation 3) lowers range on all bows and spells, a bunch of defensive spells in Alteration, Arrow Fend (air 3, Enchantment 6)... plus there are several magic items that protect commanders.

Also, any troops with shields are much more survivable against arrows. Troops with tower shields are almost impossible to kill with archers. You might try buying some heavy infantry to use as fodder for your einheres and skinshifters.