Disney Epic Mickey 2: The Power of Two

HighT3chR3dn3ck's solution is good and accurate regarding the fact that if you use Paint to get through Fort Wastleland instead of Thinner, the wall at the end of Fort Wasteland containing D.E.C. 5 (it's on the ramp up behind the projector) will not explode so you won't be able to get from Fort Wasteland to the Floatyard through this D.E.C.

However, there is a way around it as I used Paint throughout Fort Wasteland and I still managed to obtain the achievement. The wall didn't explode so i couldn't get through this way. So here's how I got the D.E.C. 5 completed:

-Go to the Floatyard through the Fort Wasteland Projector
-In the Floatyard, jump down to the mechanical robot's legs and make your way across the cliff-face (bouncing on the drums).
-Thin the clown's nose and let Oswald zap the machine so the mouth opens.
-Thin out the hinges on the Thinner sprays so you can paint the cars and jump across
-Spin the 2 cogs with Oswald to open the next clown mouth and you arrive in the area with D.E.C. 5 (it's on the left up a step).

When you get about a third of the way through the D.E.C (you'll be heading left), you will see the reason why the achievement is labelled missable. You will reach a wall which you can't thin and can't go anywhere (there will be red balls above you) so it seems you can't complete this 5th D.E.C.

Use a 2nd controller as Oswald and stand him next to you. Press 'B' on Oswald's controller so he throws you up in the air, he will throw you high enough to reach the red bouncy ball so you can continue left through the D.E.C. to the end.
The wall that would have fallen down in Fort Wasteland (if you used Thinner through the level) will be gone temporarily so you can still get out (it'll return when you leave Fort Wasteland).

The other D.E.C.'s aren't all story progression either as the one in Rainbow Caverns Devil Falls was my last one (and i'm just about to fight the last boss, i backtracked to get my missing 2 D.E.C.'s).

Here are the locations:
D.E.C. 1 - Story Progression (connects Mean Street North to Ostown)
D.E.C. 2 - Story Progression (connects Ostown - Underground to Rainbow Falls)
D.E.C. 3 - Go to the entrance of Rainbow Caverns (the cavern with the giant sword that you cross). Throw Oswald up and ride him down to the left by the lava maze. You should be standing between the lava maze and D.E.C. 3 (this is the last one i got)
D.E.C. 4 - Either navigate through the lava maze (above) or fly down on Oswald behind the projector after the giant sword in Rainbow Caverns to reach a platform with a crank. Use the Crank to open a cave with D.E.C. 4 (this will net you the Fall From Grace achievement too).
D.E.C. 5 - Fort Wastleland to Floatyard (or vice versa as explained above).

Hope this helps and is accurate. Let me know if it needs any changes rather than leaing negative votes. Thanks and good luck!

This is a miss able achievement and should be done on the play through in which you're NOT going for the "Perfectionist" achievement.

Being that this is pretty difficult to describe, the best thing I can tell you is that you'll want to just try and thin everything on this play through. If you keep thinning (objects and enemies) you'll get this achievement.

The first 4 D.E.C.'s are story related and cannot be missed. However, the 5th and final one is MISSABLE. When going through Fort Wasteland, only use thinner on the objectives (do NOT try any side quests) and enemies. If you take the friendly path here, you'll miss this achievement. The other thing to note is that this will be unlocked much later than the other four D.E.C.'s

Like I said though, on the playthrough that you're going through in which you're NOT going for the "Perfectionist" achievement you should really only be using thinner on everything anyway.