How to Kill God and Unlock Godmode
There are 6 collectable objects in total. They are on levels 10, 15, 20, 25, 30, and 40.
They are often in hidden rooms at the center of the level which will open once you complete a certain amount of the level (always behind blue force fields). Once you have all 6, hit esc and go to the God room. Now there should be a ball spawn. Throw/shoot it at God and He will die.
You can level up Godmode multiple times to make it more powerful. To do this just grab the collectable objects and kill God again as many times as you like.