There are two options for doing this easily. One involves using a ghost character if you have it unlocked, the other way is to primarily use R2D2 if you don't have the ghost yet. Neither will be attacked.
The below guide describes using R2D2. If you have a ghost use that. It is easier as you can fight and remain undetected.
The main objective is to get through the level as quickly as possible.
When there are enemies, quickly switch to a character with a gun (I would use Boba Fett because of the thermal detonator which is needed open a gate later) and blast away.
I played the whole level out like this;
At the end of the first area. I hid to the right of the stairs and moved them with a jedi. I then switched to R2 and went up the ramp. At the top I switched to Boba and killed everyone before opening the gate with C3PO.
The next area I started as R2, switched to Boba, killed everyone and then blew up the gate with the thermal detonator. DO NOT MAKE THE AT-ST. Switch back to R2 and make your way through to the gate. Quickly blast away with Boba and go into the cantina.
Once inside, switch back to R2 and make your way back outside the other end. Switch to Boba and grapple over to the next area. Kill the couple of enemies and use the detonator to blow the doors.
NOW BE CAREFUL. There are alot of enemies in the next section. Blast away and if you are losing health, blow a door or bin on the left side to get a heart. I would recommend keep jumping to get the ememies on the top ledge. When they are all gone, jump on the animal and jump up to the ledge above the doors and go down the stairs.
The last area is similar to the previous. Keep blasting away and pick up hearts if you need them. As soon as the spy is done and there are no other enemies, go into the Falcon.
The achievement is yours.
Forget all about collecting studs or canisters. Just do this as quickly as possible. It shouldn's take more than 5 mins.