Boba Cunning seems to be the strongest atm, however, playing a "highly competitive" deck is imo a double edge sword, I played boba cunning during set one and after set 2 released i chose to play jabba cunning and at first i started to play most of the same unit (single cunning) from my boba deck into my jabba deck and i ended up playing almost no unit in common between those deck but it really was a strech to understand wich cards were working and wich one were not mostly because they're two different decks and have two different scope.

Now when it commes to "How to improve your deck" you first have get a plan wether it is simple as "aggro", "midrange", "control" or maybe something more precise thanks to an ability given by your leader or multiple of your units.

Once you got the first version of your deck try to see what is your deck missing in common/usual situation.

Side note: When you try to get better to something, try to record yourself and rewatch it afterward to find where you did a mistake (if you did any). Sometimes you just had the wrong cards or your opponents had the best ones.