I think this may be exactly where many people take the wrong turn.
Its not the fixerbuffs that make you good at blitzing, its still about you, the player.
Srompu summed it up nicely, but allow me to stress out some points:
Even with GSF and HNQ your character will be put through a meat grinder if you do not know exactly how to complete your mission.
When running through all the available rooms, you’ll simply collect a comet trail of adds, some of which will take time to aim and hurt you or even snare/root you.
I consider it important to stop for a moment in the second room of a mission to get a good look at the layout.
With some experience its possible to get a good feel on where your mission objective is located (usually a large room as far away from the entrance as possible, or, on some notable exceptions, in a hidden room, so consult your PF map).
Especially in lower level missions it may be entirely possible to spot the mission objective right away.
A Find-Person-Target usually belongs to the same faction as your character and can be easily spotted on the PF map. Likewise a target for assassination will always belong to a different faction (duh) and may stick out of the normal dots on the map as well, may it be the single monster-dot or the single blue/orange dot in an mission otherwise inhabitated by white dots.
So, to sum it up, know where to go before you start!
Use an aggression enhancer to pull single targets out of a group without attracting social aggro.
Abuse line of sight as much as possible to avoid being spotted by the mobs.
A bureaucrat-related tip for blitzing: You have a pet. Make use of it!
Do not waltz into a room first, attracting everyone’s attention to yourself.
Your pet is selfcasted, weak and expendable.
It is nevertheless able to catch the attention of your targets for a few hits.
Make the pet attack an unconvenient target in your way. Let it punch the target a few times to get aggro while staying out of line of sight yourself to avoid aggro.
Now spring to action yourself and either calm or snare the mob.
Make the pet stop attacking when you cast so it does not break the calm/snare and to let its healdelta kick in.
Yes it may take three or four tries to land a calm on a nasty target, but your pet just bought you the time.
Rinse and repeat until you get to your mission objective.