Easy "Genius Economic Bad Rat"...
	Easy "One Click One Death" Ach...
	Easy "One Single Rat Move" Ach...
	Various Steam Achievements...

Start a new game, beginning from the first level, and choose the "Expert" difficulty level (because that one unlocks all rats and objects). Do not use passwords, but feel free to save the game (and load it later, if needed). Then follow this guide (which is translated from a Steam guide linked below)

Level 1 with 1 rat: Cannon Rat (rotation=38, timer=9)
Level 2 with 1 rat: Cannon Rat (rotation=0, timer=1)
Level 3 with 2 objects: Lung Rat, Wooden Board (rotation=28)
Level 4 with 1 rat: Batter Rat (rotation=34)
Level 5 with 2 rats: Batter Rat, Bomber Rat
Level 6 with 2 rats: Batter Rat, Bomber Rat
Level 7:
Level 8 with 2 rats: Batter Rat, Bomber Rat
Level 9 with 2 rats: Batter Rat, Vacuum Rat
Level 10:
Level 11:
Level 12 with 4 objects: Batter Rat, Bomber Rat, Wooden Board, Wooden Box
Level 13 with 3 rats: Batter Rat, Bomber Rat, Lung Rat
Level 14 with 3 rats: Archer Rat, Batter Rat (rotation=34), Bomber Rat
Level 15:
Level 16 with 4 rats: Archer Rat, 2x Bomber Rats, Lung Rat
Level 17 with 4 objects: Balloon, Cannon Rat (rotation=25, timer=45), Vacuum Rat, Wooden Board
Level 18 with 4 objects: Balloon, Barrel (rotation=30~40), Bomber Rat, Lung Rat
Level 19 with 2 rats: Archer Rat, Cannon Rat (rotation=47, timer=6)
Level 20 with 2 rats: Lung Rat, Vacuum Rat (rotation=89)
Level 21:
Level 22 with 2 rats: Batter Rat, Tractor Rat
Level 23:
Level 24 with 5 objects: Archer Rat, Balloon, Batter Rat, 2x Bomb Rats

As you probably know, the physics engine for this game is awful, and it may take a lot of tries until the ball decides to follow the path you wanted. So, don't give up and keep trying! And, yes, there are 18 levels listed in this guide, but you only need to complete 15 of them using fewer objects than planned.

1. Start a new game.
2. Select "Expert", so you will have all objects available.
3. Place the objects on the level. Press play.
4. Did it fail? Damn! If this is level 1, restart the game (step 1 of this guide). Otherwise, go back to the main menu and load your save. Or, if you are tired of this level, just complete it regardless of the numbers of tries (the 10 levels don't need to be consecutive).
5. Have you managed to complete it in the first try? Awesome! Now press "Next" to load the next level.
6. After the loading screen, press "Menu" at the first screen of the new level, even before you start playing it. Save your game now.
7. Press "Resume", then go back to step 3. Rinse, repeat.

If you save at different moments, the game may count additional tries; so play safe and only save at that specific screen.

This achievement can be obtained while working towards Genius Economic Bad Rat achievement. Based on the guide for that achievement, here are the levels that can be completed "easily" using only one rat:

Level 1 with 1 rat: Cannon Rat (rotation=38, timer=9)
Level 2 with 1 rat: Cannon Rat (rotation=0, timer=1)
Level 4 with 1 rat: Batter Rat (rotation=34)

Complete the indicated task to unlock the achievement.

Achievement	How to unlock
Bad Bad Rat	Forty maps solved.
Bad Rat Junior	Solved ten maps.
Bad Rats' King	Completed all levels.
Economic Bad Rat	Completed five maps using less objects than planned.
Extra Economic Bad Rat	Finished ten maps using less objects than the plan.
Fast Thinker	Solved five maps with less than one minute.
Genius Economic Bad Rat	OHH! Fifteen maps completed using less objects than planned.
Master Super Fast Thinker	Uou! Solved fifteen maps in less than one minute.
One Click One Death	Finished 10 maps in the first try.
One Single Rat Move	Completed two maps using one single object.
Serious Bad Rat	Thirty maps solved.
Super Fast Thinker	Solved ten maps with less than one minute.
Very Bad Rat	Solved twenty maps.