My usual way of dealing with Commonwealth is to declare a war, take a few provinces I can release vassals from (only one for each vassal so the truce is short) then Reconquest CB until the cows come home. Make sure to take minimum one province you can release a new vassal from in each Reconquest war, and as much money as you can to keep Poland down. This will give you some, uh, "allies", who tend to be in otherwise short supply as Muscovy, and weaken the PLC repeatedly while strengthening you.
Your greatest strength in these wars is the fact that you can have one big army, while Poland/Lithuania tends to have several smaller ones. Fight defensively at first if you don't have an immense advantage, use the winter, and push only when Pol/Lit are sat at 0 MP, or when you're confident in your army advantage.