Regarding L2 Certification

Try this: kunalsharda.com/.../

1. Learn all the topics mentioned on this page -> https://community.appian.com/learn/certifications/p/senior-developer

2. There used to be a practice test, try to take it multiple times. Once you are confident go for the certification.

All the very best !!!

Appian academy is also have a several online videos for senior developer https://academy.appian.com/