When Stan's dad shows you how to cast it, it shows a glowing green orb. When you ACTUALLY do it, you don't see an orb. It appears as a distortion. You control the distortion with the right stick and detonate with the A button. It has to land pretty close to the guards, I would put it right on the side of one of them, so that they both turn that way and you can sneak in.
Don't worry I'm very positive that that part has made a lot of people get stuck.
+1 to MS for taking 2 huge steps in the right direction. Now if only they fixed the rest of the problems :-(
You have to hover the fart over one and move it to one side and pop it. then do the same to the opposite side for the other guard. so they go in 2 opposite directions. For the guard past them, turn on the power to the wire in the water, and distract the guard to go into the water