Ultimately, for Paul, the debate with Judaism of all varieties – monergistic or synergistic – boils down to the need to submit to the righteousness of God revealed in Christ:
“Brothers, my heart’s desire and prayer to God for them is that they may be saved For I bear them witness that they have a zeal for God, but not according to knowledge. For, being ignorant of the righteousness of God, and seeking to establish their own, they did not submit to God’s righteousness. For Christ is the end of the law for righteousness to everyone who believes” (Rom 10:1-4 ESV).
All varieties of non-Messianic Judaism affirm that the law is the God-appointed means for attaining the status of “righteous before God.” Christianity teaches that Christ has brought the Mosaic Law to an end, so that righteousness is now only attained by faith in Christ.
In conclusion, I cannot do better than to quote one recent scholar who has compared Paul’s and Qumran’s doctrines of justification in detail:
“[For the Qumran sect] God’s righteousness lies in his setting the member of the community free from sin and in making it possible for him to observe the law perfectly, so that he might stand righteous before God in the final judgment on the basis of his works in the law … It is God’s prevenient grace, in establishing the covenant, that first allows for righteousness in the law; in the end, however, it is perfect fulfillment of the works of the law that establishes one as righteous before God in the final judgment …. Within Qumran’s strictly covenantal framework a nomistic basis is the only possible ground for justification, no matter how deep a sense of God’s grace is involved. Therefore justification must also be sub lege” [under the law] (emphasis mine).
[Stephen Hultgren, From the Damascus Covenant to the Covenant of the Community: Literary, Historical, and Theological Studies in the Dead Sea Scrolls (STDJ 66; Leiden: Brill, 2007), 448, 451, 459.]
Note that phrase that I have highlighted: for Qumran, the righteousness of the saved is still a righteousness under the Law, “no matter how deep a sense of God’s grace is involved.” In the final analysis, the deepest choice in soteriology is not between monergism and synergism but between Christ and the Law (as a covenant of works, not as divine revelation), between the righteousness of faith and the righteousness of the Law. To have a doctrine of justification that is truly “by grace alone” (sola gratia) one must not only affirm that righteousness is a monergistic work of God but that righteousness is “by faith alone” (sola fide) “in Christ alone” (solo Christo) apart from the works of the Law.
Did Paul misrepresent Judaism? Not in the slightest. He accurately represented Judaism of all stripes when he said that the Jews refused to submit to the righteousness of God revealed in Christ and sought to establish their own righteousness through keeping the Law. Nor is there any need to resort to the Sanders/Dunn/Wright (New Perspective) theory that, since Judaism was a religion of grace, his target must have been something else like ethnocentrism or exclusivism. Those who take this option don’t want to accuse Paul of misrepresenting Judaism, so they misrepresent Paul instead. But Paul knew his target well – the Jewish attempt to attain righteousness before God by means of the Law – and he hit it.