Trump could put on his Commander-in-Chief hat, order the wall built by the Seabees and Corps of Engineers as a military operation under the supervision of SecDef. Detach MPs and Marines from nearby installations for patrol and garrison. Gird and cover it with razor tape, trenches, mines, and machine guns with clear interlocking fields of fire, and man it up like the 38th Parallel in Korea.
Uh, pursuant to what declaration of war? The courts have stopped such military operations before. No, the best step is to take the jurisdiction over border enforcement out of the hands of the Federal courts. Hell, put the jurisdiction over ALL aliens outside the Federal courts and set up a second set of courts for handling deportations. It still takes Congress to get it done.
BTW, I’d use detained illegals to build the fence. There’s a certain justice in it.
A tiny issue of Posse Comitatus Act...