Infernax

At the title screen, input the classic Konami cheat code which is:

UP, UP, DOWN, DOWN, LEFT, RIGHT, LEFT, RIGHT, B, A, START

Or if you prefer emoticons:



After doing so you will unlock the Revelation of Judgment Day achievement and get a machine gun with infinite ammo which makes the game way easier (and you can still earn achievements too!). It's also worth noting there is an accessibility mode in the game options screen which has invincibility and infinite jump options.

I made a video showing the machine gun cheat being used if you want to check it out

Here is a video walkthrough, but if you need a written one:

1. Stay on the title screen of Infernax.
2. Type/press Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start (or Enter if using a keyboard).
3. A cutscene will start, ending with a guy saying "Lets Attack Aggressively". Achievement is yours.