Upcoming Grand Arena player - asking for advice

Don’t overthink it OP. GAC is pretty straightforward. If you have time, look at swgoh.gg to see the lists of best attack and defence teams for the past few GACs and also use their crib sheet to see what the current best counters are to your opponent’s teams. If you sign up, deploy reasonably sensibly (ie a reasonable spread across attack & defence ) and use proven counters where possible you will almost certainly win c. 50% of your battles or better - which is kind of what you’d hope to achieve. That means it’s possible to have runs of bad luck of course when you lose two or three matches in a row but that is par for the course - and most importantly you get crystals for participation so it’s never a fruitless job to just pile a team in even when they are up against it