the most fail-safe defense against assasins, even if its limited, is having scouts accompany the party. Perhaps gaining withdraw promotions only? and mobility (or something to keep it weaker than an adept which took no combat promotions)
Skelletons also work well, but they seem to be permently 1-move, and are therefore some-what inflexible, while scouts are the exact opposite when it comes to movement flexibility. The only problem of course, is that a scout still costs hammers, about the cost of a warrior iirc, which is a bout 1/3 cost of an adept iirc. Not a bad price, but still are invested hammers.
I'd say having a good number of scouts (at least as many as the # of assasins u expect to face at one time) accompanied by assasins for killing off enemy assasins, and perhaps a blur spell to keep everyone immune to first strikes, would be the most cost effective strategy which does not rely on the guardsman promotion at all.