While I've managed to reach the end of the game (at minimal health), there doesn't seem to be a key and/or mouse-button allocated to firing the gun
It's H Key for firing the gun you can also read in the help section of the game. For going into the help section there is a button on the main menu.
you can also activate abilities using keyboard by using I, z and r for invisibility, invincibility and regeneration respectively.
Regards
Harsh Raj Singhania
NexZen Studios
