I reworked the main menu so selecting the difficulty is more obvious and optimized the enemy AI.

I also added a tutorial that's available when selecting levels. 

A few more changes:

• Buffed the DMG of the Shotgun: 1.0 per bullet -> 1.2 per bullet
• Buffed the DMG of the Pistol: 0.9 per bullet -> 9.5 per bullet
• Made blood from taking DMG more obvious
• Reduced the HP of some enemies to make them less difficult to kill

With the enemy AI reworked, expect another update soon with new enemies.