Speed
Speed is your greatest ally in Doeo. To capture the creatures, you must simply position your mouse over the creature as it pops up on the level. You only have 40 seconds in each level; on easy, you need to collect 100 Doeos, while on hard, you need 200. To help increase your speed, consider increasing your mouse's sensitivity so that it moves faster and with less effort. Additionally, if you use a laptop, you may find better control using an external mouse instead of the mouse pad.
Anticipation
You must learn to anticipate where the Doeos will appear. Each level varies slightly and the level layout changes during game play. Nonetheless, you can learn to spot where Doeos may pop up. Rectangular windows in a skyscraper, for example, are a perfect spot for the sneaky creatures and Doeos regularly appear at the top and bottom of the screen. In Hard mode, learn to anticipate where the blue Doeos appear. These creatures are worth more points but do not hang around long. Missing one may hurt your score and cause you to fail the level if you do not make your quota.
Technique
Doeos follow certain patterns from level to level and learning to recognize the pattern can help you quickly rack up the catches. For example, levels often use a Doeo "train" -- a string of Doeos that travels across the level. Position your mouse over the "engine" in the train and catch the entire string of Doeos without having to move the mouse around.
Final Boss
On the last level, you must touch the King Doeo five times to pass the level. You have to employ a careful balance of speed and anticipation during the final boss stage. A circular shield surrounds the king and you must move the mouse through a narrow opening without touching the shield to succeed. Touching the side resets the level. As the shield spins, move your mouse near the opening and quickly slide past the shield through the opening when the opportunity presents itself.