with one arm tied behind (one's) back
Easily or effortlessly; having no difficulty or resistance (while doing something). Are you kidding? Montgomery is a powerhouse boxer—he'd knock that kid out with one arm tied behind his back! Oh, please, I could beat you in a game of chess with one arm tied behind my back.
with one arm tied behind one's back
Also, with one hand; with one's eyes closed. Very easily, as in I can assemble that chair with one arm tied behind my back, or I could make a better dinner with one hand, or He can do that puzzle with his eyes closed. All these phrases are hyperbolic. Also see do blindfolded.