For reference, I have only lost 3 of my grand arena matches and most often finish in first place of my group of 8. As a general rule, zetas should be prioritized to arena. After that, you should spread the love. A zeta on bossk leadership will do wonders and you may never need to add another zeta to BH again, so I'd highly suggest that. Now it's awesome that you have 4 zetas saved up, but that might also be what's causing you to fall behind. The only time I believe hording zetas is a great idea is if you are expecting something specific to come around the corner that will need zetas and will come online immediately. You always have to weigh the opportunity cost, and by saving these zetas, you've lost resources. Padme teams are unfortunately a zeta pit, but you will get a lot of value from putting both zetas on padme. Ahsoka is a great Zeta, but it's less necessary and at your discretion.
Final tip. As a general rule, I always always always look to see what my opponents roster is and prepare for the worst case scenario. If he placed all of his best teams on defense, what would I need to clear it. I save those teams for offense. The choices come in when multiple teams can win on offense, so I prioritize teams that can be undersized. For example, a bossk lead bounty hunters team is most effective with all 5 characters, so I always set it on defense, but a CLS team can work wonders with just chewie and han, and maybe C3P0, but the 5th slot is often unnecessary, so CLS works better for me on offense. If I can beat my opponents Darth revan with my jedi revan (lots of practice and a fast thrawn substitute) then I will set my darth revan team on defense in order to challenge my opponent.
Anyway, use barris with padme and you're on a good track
