- Estimated achievement difficulty: 4/10
- Offline: 29 
- Online: 0
- Estimated time for 1000: 15-30 hours
- Minimum playthroughs: 3
- Missable achievements: None!
- Glitched achievements: None!
- Cheats disable achievements: Extras disable the "Undefeated" achievements
The achievements in Lego Star Wars II aren't really difficult, with the exception of some flying levels for the "Undefeated" achievement. However to save yourself the trouble of having to search for all of the secret items in the game, use a walkthrough.
I found THIS walkthrough particularly helpful.
Preparation: Finding the right red bricks
Every level has a red power brick that unlocks a certain extra that you can purchase from the counter in the cantina. Getting the Multiplier extras and purchasing them immediately will make a huge difference. So, until these become available, DON’T BUY ANYTHING!!! Use the power brick guide below to find the locations and decide if you can get them in story mode, or if they’ll require a quick free play before moving on.
Score X2 – Power Brick 6 (Rebel Attack) – 2,000,000
Score X4 – Power Brick 9 (Falcon Flight) – 6,000,000
Score X6 – Power Brick 12 (Bespin) – 12,000,000
Score X8 – Power Brick 15 (Speeder Showdown) – 20,000,000
Score X10 – Power Brick 18 (Death Star II) – 30,000,000
The first couple can take a while to save up for, but once you get them, keep in mind that they work in tandum. If you have X2 and X4 on at the same time, you’re actually receiving X8. If you have X2, X4, and X6 on, you’re actually receiving X48, etc… So once you get the first couple, you’ll be raking in studs at an alarming pace. Resist the urge to waste money on additional characters. None of the characters available for purchase (except ghosts) have any abilities that the free ones don’t already possess, so just skip buying them until you have all the power brick multipliers. Once you have all the multipliers just buy everything. It will hardly take you any time to collect enough studs, when you’re earning X3840.
Step 1: Story Mode.
Play through all eighteen chapters in Story Mode and try your best to fill up the True Jedi meters in each level to save yourself the trouble of having to replay them later.
Step 2: Free Play Mode.
Play through all 18 chapters in Free Play Mode and collect every Minikit (Ten in each level) and every Power Brick (One in each level). As you progress through, make sure to purchase some helpful extras such as Invincibility, Mini-Kit detector, Multipliers (x2, x4, etc), etc. Also make sure to fill up the True Jedi meter in Free Play.
Step 3: Collecting the remaining Gold Bricks.
In each Episode there will be a bonus door which contains three stages (two of which, you must collect 1,000,000 studs before the time is up, and the other is a Super Story, which requires you to play through every chapter in that Episode in under one hour (or collect 100,000 studs)).
Now you will need to buy every Bounty Hunter from the shop and head through the Bounty Hunter door inside the Cantina. Here you will need to find and make contact with, ten different characters in order to earn the Gold Bricks.
To finish off collecting the gold bricks, purchase the final eight from the Cantina Shop.
Step 4: Lego City.
This level is simply a bonus level required for the 100% achievement. Head outside the Cantina and find a pile of Gold Bricks which you can make into a question mark with a door at the bottom. Head inside and you will need to collect 1,000,000 studs. You don't have a time limit so take your time and get collecting!
Step 5: Purchase Everything!
Now, before you work on the "Undefeated" achievements, head outside the Cantina and build the pile of Gold Bricks into a Stud Fountain. Keep collecting studs and purchase everything in the shop (starting with the multiplier extras to make it easier). Once you have bought everything, the 100% achievement should unlock.
Step 6: Undefeated.
The final step towards your 1000G are the "Undefeated" achievements. To begin with, complete all of the missions that involve being on your feet and not in a vehicle by using one of the Ghost Characters who can't die (unless you fall down a hole, or get shot out of a vehicle). Once these are all completed, you will be left with the flying levels. These can be quite tricky but with a bit of practice, combined with some skill and luck, you should unlock these eventually.
Well, that's about everything, if you followed this then you should have the full 1000G, so congratulations!