Here's the idea: big sets on the barbell, and chip away on the gymnastics portion. It's going to be a lot less energy expended to reset on the rig than it is to reset on the barbell, so make your money on the barbell, recover on the rig. Besides that, the good news is, if you don't finish 21-15-9, you get saved by the clock! If you DO, then even better news, you get to keep going until the clock says so. Nothin better on a Monday than some good ol' fashioned CrossFit!