Try to get as close as possible to the enemy so more of your shots have a chance to hit without scattering. Also I highly suggest becoming really good with the pistol, so you can take out weakened enemies without wasting precious ammo, and it is much more accurat over a longer range, use the blast cannon for short distance targets, while using the pistol for medium and long range targets, it's no sniper rifle but at least most of the shots will hit.
If your the dark trooper use the jetpack to get as close to an enemy as possible so your scatter shots deal more damage, just be sure not to get to far ahead of your friends as a lone darktrooper will be easily shot down. Smaller city like maps are great for the darktrooper as he can easily bypass some obsticals that the normal trooper would have to fight to get around, try going to the furthest back and hopefully least guarded command post. Once there make sure you stand facing the command post, with your back to the wall, try standing in a spot where the enemy can't sneak up on you, if anyone spawns try to take them down with one shot, if too many people spawn you can use your jet pack to get away and wait for the coast to clear.
As for the rebel pilot, well I haven't really used him much, he doesn't have the jet troopers advantage at being able to jump over the enemies, really I only use him when I'm planning to use a vehicle, but if you want to use him for battle I suggest using him as a back up troop, support the soldiers with your pistol and hand out health and ammo, if the enemy gets to close pull out your shot gun.
I hope that helps, I don't really use the blast cannon alot but I have seen plenty of people become exellent at it.
Not being paranoid doesn't mean someone isn't trying to kill you... or does it?