Leedmees

If you plan on going for the full 200/200 in this game, you should probably first focus on beating all the levels and attempting all 'S' ranks first as this is a cumulative achievement .

If you decide you want to speed up the process, the fastest stage to net a bunch of Leedmees is Stage 22.

In this stage, Leedmees fall from the sky in bubbles and you just have to stand under them and catch them. Hold your arms up like a field goal to help catch as many at a time as possible without the fear of them walking off to their deaths.

Once you scoop up a bunch, just keep making trips to the Red goal point (which moves constantly in this stage).

It is a very wide open level with very few reasons to lose any Leedmees. There are 24 total on the stage, and it can be beaten in about 40 seconds.

Keep repeating the process and you will eventually get there. As far as I know, there is no stats page to count your Leedmees. I beat all 50 single player levels and achieved 'S' rank on every one, and it still took about 20 minutes of the above method for the achievement to unlock.