The catch is that you have to know what you're doing, heh. So you'll have to learn how each monster's tail behaves, so you can connect the boomerang more. Once you know how to do that, expect to get tails within 5 minutes (for the easy to hit ones). Some monsters will probably require you knock them down. And remember that some monsters require their tail to be broken first before you can cut it (Narga and Uragaan; Agnaktor and Barroth need to have the ice/lava/mud armor removed).