Although I have passed the Oracle Java Associate exam, like you, I am also preparing for the professional exam, so, for what it's worth.
Patience, read the material, variety of mock exam providers and undertake a substantial number of mock exam questions.
There seem to me many mock exam providers. Google searches reveal sites like · CertPal, Exam EPractize Labs, Dan Chisholm's Test Suite, Marcus Green's Site, Whizlabs Java Certification Kits, JavaBeat, Certification Kit and the list goes on. I've worked mostly with B & S, Whizlabs and CertPal.
The B & S questions seem to be consistently the most difficult, partly because they often require more than 1 correct answer. How that level of rigorousness relates to the actual exam, I'm not sure.
I've found it helpful to document my incorrect answers so I can revisit them numerous times until I feel more confident in my understanding of them.
Ive also found it somewhat helpful to keep track of my scores so I can notice any progress when I retake an exam a month later.
Also, depending on your background, this undertaking could take a while but, presumably, it will be worth the investment of time.