Bespin's Cloud City is a great place to practice with the Dark Trooper cannon. There's lots of little platforms and ledges you can jump to and get really close.
I had the same problem, I'm wasting ammo taking 4 or 5 shots to kill someone, when I realize a pistol will do the same. But, when I get right on top of an enemy, 1 shot will do it. The cannon can be frustrating, but practise, which I'm finally getting, will help.