You don't get it. If an otherwise Constitutionally valid law is to be enforced in some unContitutional manner, then it is the enforcement that must be enjoined -- as a violation of the Constitution.
If by "unconstitutional" you mean "not explicitly mentioned in the Constitution" then this is a bizarre argument, precisely because the Necessary and Proper Clause is the bit that
gives Congress the power to enforce laws made in pursuance of other enumerated powers. Perhaps you could lay out for us what you consider to be Constitutional enforcement mechanisms? (As the Marshall Court noted so many years ago, if you read the Constitution the way you're apparently reading it, the federal government can't actually punish people for breaking most of its laws, aside from the few areas in which punishment is explicitly mentioned in the Constitution, like in cases of counterfeiting and crimes on the high seas--do you really not believe that the federal government can legitimately punish you for stealing mail?)
Regardless, this argument was considered and
settled over two centuries ago. Or as Madison noted in the Federalist papers:
Had the convention attempted a positive enumeration of the powers necessary and proper for carrying their other powers into effect, the attempt would have involved a complete digest of laws on every subject to which the Constitution relates; accommodated too, not only to the existing state of things, but to all the possible changes which futurity may produce; for in every new application of a general power, the particular powers, which are the means of attaining the object of the general power, must always necessarily vary with that object, and be often properly varied whilst the object remains the same.
They didn't write 'em down, but they're there.
What ENUMERATED power permits Congress to tell me that I am obliged to buy ANY insurance?
The argument being made by the government in this suit is that the power to mandate coverage is necessary and proper for carrying out its (unchallenged, even by Virginia in this case) power to regulate health insurance under the Commerce Clause.
The argument that N&C doesn't apply because the power that the mandate is being instituted to carry out is unconstitutional simply fails, since everyone in this case was in agreement that the insurance market regulations are Constitutional.