Remember that you can't fire the Cannon while in air. Thus, you can forget hanging with the hook (if the map allows) and firing. Also, in the latest versions of TF, the swimming and firing Hwguy is no more. Your feet have to be firmly planted on the ground.
When firing against moving targets, try to predict their movements and have THEM run into your bullet stream. It may also be a good idea to sweep the area, in order to catch those enemies low on health. Watch out for your teammates, though. You don't want to wear away all of their armor.
The major weakness with the Hwguy deals with his movement while using the Cannon. Rather, the lack of it. You make excellent ANYthing-bait while firing the cannon. You can pretty much expect hell to come your way once you start firing, as you are literally a sitting duck.
In general, you don't have to be too afraid of charging into rooms or whatnot. You start out with more armor than anyone else, certainly enough to get you through a few meagerly placed grenandes or rockets. (It usually takes 5 perfectly-placed rockets to take out a Hwguy, so you can expect a reload) Of course, it's still best to practice strafing and all that, since you're likely to encounter more than one enemy at a time. As for the MIRV grenande, use it wisely. You start out with only one, so don't waste it.
The Hwguy is obviously best suited for defense. Force enemy players to come to you. Park yourself into a nice corner, so you can't be circle-strafed, and camp away. Of course, watch out for grenades that are likely to be bounced into the room that you're in.