I'm no expert, but I'd definitely recommend aiming for the head region. When you pull on the line the fish's face will turn causing the whole fish to turn and tie up in the kelp faster. If your shoot it in the back section of the fish you will have no leverage to change the fish's direction. It will just keep swimming straight until it takes all of your line. That and a head shot has more meat/structure to hold your tip so there's less chance your shaft will pull out.
I also use a carter float tied to the end of my gun. After the fish ties up I inflate the float and then go find my fish. It's nice to be able to look back and know exactly where my gun is.
And I use a reel, but that's just how I roll.