Egglien (EP)

This is very easy achievement. Just beat the boss of world 1 and most likely you will beat him in less than a minute. Wait for him to come down a bit jump and shoot. When he loses about 30% of his health he will go up and start shooting lightning so make sure to jump back down and avoid it. Repeat this process 3-4 times and you are done!

Here's video walkthrough with all achievements (Megacloud boss fight starts at 4:48):

Timestamps in the pinned comment for all levels.