It seems Arceus tests her own trials before forcing Mew and other legendaries to go through them. To make it a fair and legitimate evaluation, she changes her biology to need air and to be able to hold air for 30 minutes, the current longest time for her legendary subjects. After, she seals her power away in a magic boxv that releases the magic back to her when she drowns completely.

Sketch 1: Arceus calmly starts testing a trial. This trial begins with a long, labyrinthian set of hallways.

Sketch 2: Arceus finds the door to the next section, grabbing the door valve and twisting it.

Sketch 3: The next hallway is blocked with many rocks and boulders. Arceus gets to work, squatting down and lifting rocks out of the way.

Sketch 4: At the end of the hallway was a locked for. A stupid amount of keys are Floating around, with no indication as to which is the right one...