Both tests are very tough. Neither is going to be passed by rote memorization as many of the questions require analysis of system output before trying to adjust for the desired result. There are no true or false questions and no essays. What you will get is a series of questions written by subject matter experts that range from testing basic conceptual knowledge to very tough assessments of how to fix problems. BTW the exam software lets you mark questions for later review and it does help to bypass the tougher questions for later review at the end of the test.
Are the question tricky? No, not really. They really require in depth knowledge of MySQL 8.0. If you have ever interviewed someone for a DBA job you know there are certain points of knowledge (pain points) that a candidate MUST have to be trusted with your precious data. Imagine going to your physician and having to identify which body part is a knee and which part is an elbow. I personally would trust anyone with either the 8.0 DBA or 8.0 Developer certification to know their stuff.
Certification guide? Nope, sorry. Back in the 5.0 days there was a team effort to produce a comprehensive exam guide. But 5.0 was a long time ago and there was an effort back in the 5.6 days for a resurrected guide that did not come to fruition. And having written a technical book, I can assure you that any such work is like writing in sand on a busy holiday vacation spot beach as the content is constantly changes and the community is adding or dropping tools continually.
I highly recommend Oracle's classes like MySQL For Developers and from regular reviews of the course materials on my part tell me that the curriculum team does a fantastic job conveying the complex material way in an easy to consume package of literature and exercises. I still recommend practicing the exercises for several weeks after the class before taking the exam.
Exam crams? Do not waste you money. The 'dumps' I have examined in the past are either poor third-hands recitations of poor interpretations of what was on a test or bad copies from the manual.
How to Have a Good Chance to Pass the Exam
Review the exam topics below, using it as a check list. Go to the MySQL Manual enter and read the sections for that area. Make sure you understand any examples and please do try them on your own scratch server. Please write down notes in a notebook if you think having a consolidated review tool is handy for you.
Also check the MySQL Server Team blog and Planet.mysql.com for blogs from other Oracle MySQL Teams.
And if you have questions please ask away on forums.mysql.com and mysqlcommunity.slack,com. I monitor the Certification board on the forums if you have questions
The Exam Topics