Some good comments from Calvinist Michael Bird, admitting that Calvinism (not just Arminianism) involves synergism (in the context of talking about Universalism):
Calvinists like to tout themselves as holding to a form of monergism whereby God alone works salvation in the individual, while those horrid Arminians and Catholics purportedly teach a synergism of divine and human wills. The problem is that any system of theology, including Calvinism, that recognizes a tension between divine sovereignty and human responsibility is going to entertain some form of synergism. Unless humans are nothing more than puppets there is always going to be the objective work of God countenanced with the subjective response of humanity to the divine work.
In the Reformed scheme human will is freed and faith is activated by the regenerating work of the Spirit. God takes the initiative, he is utterly sovereign, his purposes are assured, but I’d hardly call it monergism in the literal sense. Truth be told, the only true monergism is universalism since God alone does everything for salvation, no response, not even faith is required, and there simply is no tension about divine sovereignty and human responsibility on the universalist scheme. Understood this way universalists are the true “Calminians,” a hybrid Calvinist-Arminian offspring, as they combine the Calvinistic view of the efficacy of God’s saving power with the Arminian view of the universal scope of God’s salvation. God’s love is universal and his power is limitless; what God desires must effectively come to pass. If his desire is that all people be saved, then all people must be saved. However, this is a jaundiced view of salvation.
Taken from: http://euangelizomai.blogspot.com/2011/03/reflections-on-universalism.html.