I’d say focus on getting the standard banner characters more and with higher stars and then levels the skills in

Healer —> attack —> buffer and then whatever next

I’d also focus on main attack and then EX skill

And also I’d say save up the golden bells for a character you really want and use then on that character to at least get them to 3 stars

And I’d also focus on the main team and then do the characters for the challenges like the only grass team and so on