Don't worry, it's a bit hard to beat. Leave it in a lost folder until you feel like trying again.
It takes a bit to get used to the enemy patterns and placement, but they are very predictable so it should get easier after a while.
The main thing going on for this game is that you're only a human fighting very powerful creatures, but you're an expert on what you do, so using your subweapons correctly can make a huge difference. Take a look at this gameplay video about a guy making full use of the subweapon system (and cheesing the boss)