As long as he prefaces the assassination order with "In my constitutional duty to defend the Constitution from all enemies, foreign and domestic..." before he adds "... I hereby order the assassination of whoever the hell I want."
Then it's all buttoned up! Thanks, SCOTUS!
1. The military, and any other part of gov't currently committed to not carrying out illegal orders, would refuse them.
2. The corrupt court decides what actions are and are not immune from prosecution.