I too found the ASA nav test more difficult than the GC license test.
There's alot of fill in answers, so no multiple quessing.
If I had to offer one suggestion, it would be to make sure that you read the question correctly and make sure you know what is being asked for.
There's no time limit..so take your time.
Be very comfortable with Set and Drift problems.......and 60d street...Speed/time/distance.
Know how to convert up and down ...through TVMDC ...remember...that **currents are always expressed in T
Measure twice...bring good nav tools..a proper pencil...# 2 is too fat!!
Know how to convert time from decimals...... 1/10 of an hour is 6 minutes or .1
Sounds easy, I know. Remember to use the latitude scale...not longitude..
Common, mistakes...that I see.
The best book, that I used to prepare for both CG and ASA nav, tests was Mike Pyzels. I believe ASA has started using it again...worth the investment imo.