契約者=contractor
よ=vocative case marker
我が=my (the archaic form of 私の)
盾=shield
と=the case particle
なり=ren'you-kei(連用形) form of the verb "なる/to become"
剣=sword
と=the case particle
なれ=the imperative form of the verb なる
Contractor,
Be my shield and sword.