I´m surprised! Shield wall subtracts -6 from missile weapons´ damage and -8 from the shooters´ MW. Together with the heavy armour of the Guard that´s pretty impervious except for the odd lucky roll. Hail of arrows would negate the MW mod, but assault warriors are not able to loose a Hail.
Of course, unsupported Iron Guard out in the open ground won´t last long, but that´s what the auxilia is for, right?