Nope, I was kinda stuck there as well for a few minutes. In on of the corners of the cargo hold is a switch that opens the back of the plane. Then you can use the 4 levers around the big ball. Pretty ez when you find the switch

Shoot the 2 chains that hold up the upper level. It will then smack down, and you can go up on it. The chains are yellow if i remember right

There's a switch on that plane on the wall somewhere what will open the planefloor.

There's a devil when i killed the laserboss.. How do i kill the devil??!

I've activated a console, that moved a bridge to the ball thing. But I can't do anything else.

The bridge is released, you run up to it, walk to the sphere, press the use button, there's a little thing sticking out of it. After it's used, the sphere rises and you can hack an other security terminal. Then you repeat the whole thing two times. And that's it.

Just teleport behind him and hit him in the back with an infernal attack.

What do I do after iv got this blue ball up in the air

When you activate one of the switches next to the demon, the "blue sphere" appears, and a platform appears too. Just climb up the stairs, walk on the platform and press activate when you're next to the blue sphere.
It will activate something in it, then the blue sphere will dissapear.
Now go down the stairs, find another switch (next to the demon) that you can activate (look for an orange light in it), and repeat the sequence.
Do this 3 times...but I must warn you: the ending is really really lame ^^