probably the most helpfull tip for making decaps and disemberments at all is that u should aim for the joint. ( the round thing which u change to move.) to make it easier.
as example if u want to make a decap. try to hit the neck. u need much less power for a decap then simply kicking his head.
same for booms.
take a look about the default replays in toribash, there are some nice booms and decaps.
or take a look into the replay section, im sure there are alot of people which post booms and decaps! and even much more, like parkours, tricking, skeetshots and all the other stuff u can do in toribash ;)