Is there any good add-on creation tutorial for xenforo 2.x aside from the ones featured at the official site? There seems only few tutorials available and most were not beginner friendly. What php and sql skills are required for such task?
Tutorials you can find online, and regarding your question which skills you need regarding php and sql... I guess the answer is simple, if you want to make it work with the Xenforo you're using, you need to write the php and the sql in the right php version that your Xenforo is using.