Letting the ninja loose at the crates/barrels is a great way to passive-play if you have some spare crates/barrels or don't mind sparing some game coins (make sure you tap all the crates/barrels you have placed out so that the ninja will get to all of them on his own subsequently - also the ninja ai tends to register the crates/barrels as broken right before the hit that actually breaks it, so keep an eye out for this as you will have to tap it again to get the ninja to finish it off). Meanwhile you can amuse yourself with something else or set up the next activity for the ninja. But if you want the job done quickly, you can skip the time the ninja wastes tripping over everything, getting jammed between things and kicking things out of reach. Pin him down with one finger, and use another finger to beat he ninja with the crate/barrel. This is helpful for when you're trying to get through tasks quickly. You'll need to wait a second or two between each time you hit him because the game only registers something like one hit a second. You might need to be a bit slow with this so that you don't lose your grip on the crate/barrel.