Congress gave Obama the power to declare any citizen a terrorist and to hand such citizens over to the Department of Defense so you're kind of off the mark on that one. Now, the execution without trial is an Obama Production all the way, but you aren't going to find very many members of Congress that will do anything to about it. So, again, a bit off the mark here.
More generally, though, the statement that the president's "sole purpose is to execute the mandates of Congress" is wildly off base. It may have been true at one time in our history that the Presidency was a ministerial position (though I don't think it is), but it is hardly true of the modern presidency both because Presidents over time have exercised more and more authority than in the past and because Congress has delegated shitloads of authority to the Presidency that Congress itself should actually exercise. The modern Presidency has lots of authority to do lots of things and a President that wants to exercise that authority "aggressively" can certainly do so without getting "more power to get around Congress." The President can't do everything without Congress, but he can do lots of things.