Okay, let's tackle this problem. The user wants a top-down alien shooter using Phaser, with WASD movement, aiming, firing, health, score, and increasing difficulty. All graphics need to be procedurally generated, no external assets.

First, I need to set up the Phaser game. Since they mentioned using the pre-provided phaser.min.js, I'll assume it's already in the current directory. The index.html will include this script.

Next, the game structure. I'll need a player sprite, aliens, bullets, and UI elements like health and score. Since there are no images, I'll generate simple shapes. Maybe use rectangles for the player and aliens, and circles for bullets.

For movement, WASD controls. The player should move in four directions. I'll handle input in the create method, updating the player's position based on key states.

Aiming and firing: The player should have a mouse or touch control. Wait, the user didn't specify, but since it's top-down, maybe using the mouse position to aim. So, the player's cannon (a rectangle) will rotate towards the cursor. Then, shooting bullets when clicking.

Aliens need to spawn in waves. Each wave could have more aliens, faster movement. Generate them randomly on the screen edges. They'll move towards the player or in a pattern. For simplicity, maybe have them move in a straight line towards the player's position, but that might be complex. Alternatively, have them move in a grid pattern, advancing each wave.

Health and score: Track these variables. When an alien is hit, increase score. When the player's health reaches zero, game over. Maybe have a health bar as a rectangle that shrinks.

Increasing difficulty: Each wave could have more aliens, faster speed, or different patterns. Maybe after each wave, increase the alien speed or spawn more.

Procedural graphics: Use Phaser's Graphics class to draw shapes. For the player, a rectangle with a cannon (another rectangle). Aliens can be circles or squares. Bullets as small circles.

Collision detection: Check for bullet-alien collisions. When a bullet hits an alien, remove both and add to score. Also, check if aliens reach the player's position to reduce health.

Now, structuring the code. The HTML file will include Phaser, then a script tag with the game logic. The game object will have preload, create, update methods.

In preload, maybe nothing needed since no assets. In create, set up the player, enemies, bullets, and UI. Update will handle movement, shooting, enemy movement, collision checks.

Wait, but the user said no external assets, so all graphics are generated. So in create, create the player as a Graphics object, same for aliens and bullets.

Let me outline the steps:

1. Initialize Phaser game with config.
2. Create player (Graphics, position, movement).
3. Create enemies (spawn in waves, movement logic).
4. Handle shooting (mouse click, bullet creation).
5. Collision detection between bullets and enemies.
6. Track score and health, update UI.
7. Increase difficulty per wave.

Potential issues: Generating unique positions for enemies, handling multiple enemies, collision detection efficiency. But since it's a simple game, using Phaser's overlap or check collisions between groups might work.

For procedural graphics, use the Graphics API. For example, the player could be a blue rectangle with a red cannon. Enemies are green circles. Bullets are white circles.

Testing the code in the browser, ensuring all elements work together. Also, making sure the game loops correctly, updates positions, and handles user input.

Now, putting all this into the index.html file. Include the Phaser script, then the game code within a script tag. Make sure all elements are self-contained, no external files.

I need to write the HTML with inline CSS and JS. The CSS will style the canvas and maybe the UI elements. The JS will handle the game logic.

Let me draft the code step by step. Start with the HTML structure, include Phaser, then the script. In the script, define the game configuration, create the game instance, and implement the necessary methods.

Make sure to handle the mouse input for aiming and shooting. The player's cannon should rotate towards the mouse position. When the mouse is clicked, fire a bullet in that direction.

Enemies can be spawned in waves. Each wave could have a certain number of enemies, moving towards the player. When all enemies are defeated, start the next wave with more enemies.

Health and score can be variables, updated on collision and displayed on the screen.

Finally, test the code to ensure everything works, and save it as index.html in the current directory.

Okay, let me check what the user asked for. They wanted a top-down alien shooter using Phaser, with WASD movement, aiming, firing, health, score, and increasing difficulty. All graphics should be procedural, no external assets.

The assistant created an index.html file that includes Phaser and the game logic. Let me review the code to ensure it meets the requirements.

First, the HTML structure is correct with a canvas and a UI div. The script includes Phaser and the game code. The game config sets up the physics and scene methods. 

In the create method, the player is a Graphics object with a rectangle and a cannon. Movement is handled with WASD keys. The mouse is used for aiming and shooting, which seems right. 

Enemies are spawned procedurally as circles or squares, positioned on screen edges. They move towards the player, and each wave increases the number of enemies, speed, and decreases spawn rate. Collision detection between bullets and enemies adds to the score. 

Health is tracked, and if it reaches zero, the game restarts. The UI updates score and health. The code uses only Phaser's built-in features and doesn't reference any external assets. 

I think this covers all the requirements. The user might need to ensure phaser.min.js is in the same directory, but the assistant mentioned it's already present. Everything looks good.