The darktrooper was ganked before the game even came out :P
The jetpack is a poor mans version of the clone one and the gun is the worst in the game, I use the pistol more than I do his main gun when playing as the dark trooper.
But for using his blaster:
Try to use the jumppack to get into close/point blank range, this isnt too hard but don't expect to be jumping out quickly as it has a very slow recharge.
Hit the enemy at point blank, this is hard with a dodging enemy, even a close range shot may not kill. With the movement and screen limitations of a game this is a lot harder to do than in real life.
The dark trooper blaster is a bad weapon, its a shotgun without a shotguns biggest advantage - stopping power.
The only time I have mannaged to kill anyone in one shot with this is a point blank (1-2m or less) and in the head.
It has a slow recycle time for the shots so with your extremely limited life expectancy in close range don't expect to get too many shots, it also has only 5 shots before it has to be reloaded making it even worse.
Jumping out at 1 or 2 enemys is probably the best tactic, don't expect to be able to jump into a group of them, even with surprise on your side the low power, slow recycle of the shots, limited magazine and the difficulty of hitting even with the shotgun style spread will have you on the scrap heap in no time.
Aliens sh** me - Emperor Palpatine (attributed)