Let them go into recess.
Then, the president can do recess appointments which, I believe, are valid for one year.
Democratic obstruction against nominees is nearly total, most notably including a demand for cloture filings for every nominee—no matter how minor the position. This means a two-day waiting period and then another 30 hours of debate. The 30-hour rule means Mr. Trump might not be able to fill all of those 400 positions in four years. The cloture rule also allows the minority to halt other business during the 30-hour debate period, which helps slow the GOP policy and oversight agenda.