For instance, the Hoise made up, out of whole cloth, the concept of "Obstruction of the House" when subpoenas were ignored. Under the separation of powers, the House should have sought relief from the Judiciary, but instead are making up the charge of Obstruction of the House. If the court had ordered the President to respond to the subpoena, and he did not do so, then perhaps the claim might stick.
So how does one plead "not guilty" to a scheme crafted by the Democrats where the legal construct is so flawed that it is nonsensical?
It's not a question of answering as though a crime is alleged. An impeachable action is alleged. Just answer why it is not impeachable. House didn;t seek court assistance to compel, so conclusion POTUS would not comply is premature. Assert privilege, etc.
The decider in this case is the senate. Just argue to the Senate why it is not impeachable, then they decide.