So you´re hunting down the "Madness"-Achievements?

First things first. It might be counterintuitive but in case you have already defeatet Leviathan, you might want to start all over again. Don´t worry. As long as you haven´t reached Lv35 or higher you won´t lose that much time. In case you actually have reached let´s say Lv 45 and can´t use the faster methods I discribe in the sections below there are two other ways you can use depending on the color of your sparkle.

You have already progressed too far with a blue sparkle? Great! At least it´s not that bad.
Go to level selection screen and choose "Sunny Shore". Once you started the level, go far to the right. There will be a pretty much empty space with six jellyfishes.
Gather them all and use your siphoning ability to kill them all at once and collect all the elements they leave behind. You will gain 14 elements per jellyfish (84 if you kill all 6). Done? Good! Load Sunny Shore again and repeat the jellyfish-hunt until Lv50.

You don´t have a blue sparkle? It´s a red or green one? Ouh... In that case I recommand clearing Sunny Shore over and over again. It has only two layers and only a few obstacles but many easy to get elements. Stil, even if your sparkle has a high level, starting a new save file is an option, you know?

Now we have the opportunity to spend some time with our old buddy: the jellyfish. What? You don´t like them? They are annoying? Even better! Cause you are going to ruin their day before they even get born - you monster.

Remember; every element you collect counts as 1 Exp. However; the eggs in a jellyfish nest provide you one red, blue and green element per egg. But that´s not all! When it comes to these eggs, your sparkles absorbtion speed is completly irrelevant. They get absorbed instantly. Sadly they only spawn during the "clear jellyfish nest"-quest and exactly this is the reason why you may had to start a new save file.

Important! Don´t clear all 3 nests in a quest! If you do, they don´t respawn after reloading the level. The best spot to start grinding is (again) Sunny Shore. Once you startet the quest you have two options. Go from your starting point to the left
or go to the right

It doesn´t really matter which nest you pick. Once you´re there, kill the three jellyfishes who guard the nest. You can ignore the elements the jellyfishes leave behind. What we want are the eggs. Clear one or both nests and load Sunny Shore again. Rinse and repeat until Lv50.

If you have already cleared the quest in Sunny Shore but havent progessed too far, you can also grind in Deep Blue. Here you can find another "clear jellyfish nest"-quest. Just swim down a layer and head to the right side of the map.

Everytime you clear a nest, you will collect about 100 Exp. You will need 8500 Exp in total to reach Lv50. Sounds like much? Not really. Of cause it takes some time but this method is still way faster than collecting every element one by one.

That said; have fun and go get those achievements! :)