Barrel drooping is very common in Break Barrel Air rifles.
Step 1: Basics:
Step 2: Shimming
A shim is anything that can prop the scope into the position desired.
For this I recommend using card stock, sheet plastic, aluminum sheeting (soda cans) any one of those will work just fine as shim material.
You will need scissors as well to cut the shims.
Step 3: Starting It Off
Note: each shim will raise the scope a touch to help offset the adjustments on the scope.
Note 2: you may need multiple layers of each shim.
Step 4: Final Steps
Good thing about this is that when done properly it will not damage or require you to modify your scope and can easily be undone.
Feel free to comment any questions.