How to beat 7-15X.
This level is simple, but execution is difficult due to the randomness of the demon maws' movement and spread projectiles. All you have to do is bounce off a wall (either will do since the room is symmetrical), grab the key in the middle, then survive. The easiest way to survive is to hide behind the blocks in the upper corners.
- Get the key
First, run towards one of the walls. About half way between the center of the room and the wall, jump. Wall jump off of the wall and move upwards, landing on the ledge that is sticking out of the wall. Immediately launch yourself towards the key. Let go of the run button at this time so you don't miss the key, then hold the run button as you continue falling to make sure you avoid any maw projectiles.
- Get to safety
Continue running in the same direction you were when you reached the key. Head for the far wall, then repeat the wall jump technique. When you get to the wall that sticks out one square, do a close wall jump to get up above it. Immediately jump behind the isolated floating block.
If done correctly, the last batch of three maws will explode on the ground at the bottom and you won't get hit by any of the projectiles. Then, as you jump to hide behind the isolated, floating block, the middle maw portal will send out a maw directly at you, but you will be behind the block so it will explode on the block.
- Wait for the lock to open
Now all you have to do is jump every ~2 seconds to hide behind the floating block. Every time the middle maw portal launches a maw, be sure to be behind that block. If done correctly, the middle maw will explode on the closest block. Otherwise, it will get close to you and maybe even hit the block in front of you. Make sure you do not jump all the way up into the upper right corner (this feels easier because you can just hold the button all the way). This is dangerous because the maws can target you in the corner and if they do, they won't hit the blocks; instead, they will explode on the wall beside you and you won't be able to dodge (it there are about 3 squares where you could be safe at this time, but the likelyhood of avoiding it is slim, and the maw portal above you will likely reach line of sight and send out one straight down on your head).