For those who have problems with this achievement, in many levels, there is a method I used to easily get 100% with the Mighty Eagle.
As devils Bayou said, try to clear maximum of the level with your available birds before activating Mighty Eagle, especially parts of the level you identified not to be on Mighty Eagle path
BUT and that is the trick
Launch your birds such that they will be on the path of your Mighty Eagle (depending on the birds type, you will be able to keep 1 or 2 birds "alive")
Quickly switch to the Mighty Eagle and launch it BEFORE your birds disappear.
You will then see your Mighty Eagle acting like on a pool table and pushing your birds with an extraordinary speed.
By this way you have some extra % which prevents you to clear the whole level if you don't use this method... And for example, even yellow birds can easily destroy stone elements, which is impossible if used alone.
Unfortunatety, this method does not work the Mighty Dragon in the "Year of the Dragon" episod. This one is really "Pain in the ass"
Hope this will help you to get this achievement in an easier way.
- Pigs or Birds (which are your main target to finish a level) are automatically cleared when you launch the Mighty Eagle. So don't care about them.
- You don't need to clear completely a level to get 100% in many levels. But in some, it is mandatory. With my method explained above, it is no longer mandatory OR it is easier to clear the level.
- Be careful, on your way to clear a level, if you clear all your targets (pigs or birds), the game let you a short, very short extra time during when you can launch other birds in order to keep activity and destruction in the level. By this way, your 100% target is still "available". If the Mighty Eagle Icon disappear, it is over and you have to restart your level. As soon as you have activated your Mighty Eagle, there is no time limit. You can keep your time to aim where you want to launch the Mighty Eagle.
- It seems that % given for stone parts are greater than for wood and other parts. So try to priority destroy stone bricks.
- Some other elements like baloon give extra %, so do not overlook them...