I find brawl ball really useful to push trophies. On my RTPIM account, all 3 of my brawlers at 500 are with Brawl ball. On my main account, I still play a lot of brawl ball. Shelly is really easy to get past rank 23 in brawl ball since on a lot of maps, the shelly wall destroying trick is very hard to be stopped. Solo Showdown prevents Shelly to reach her maximum potential since it is more 1v1 combat. The thing about bounty is that it is a lot more teammate reliant that Brawl Ball. Another Gamemode I prefer grinding trophies on is Duo showdown. When you are playing power 1 brawlers at the higher trophy ranges, you need a good teammate and powercubes. An issue with brawl ball is that when you play a low damage brawler like poco at power 1, you need powercubes to get him to shine. Another thing one would need to know is that maps matter. Many showdown maps favor one brawler over another. For example Cavern Churn favors Bhelly, and Rosa, Feast or Famine favors BiBi or Rosa, Thousand Lakes favors Rosa and a lot of other brawlers, and many more.
Instead of using the rank system, you could use a total trophy system. For example, a 500 trophy brawler divided by 10 is 50, which you divide by 10 again, since there is another 0 at the end, and you get 5, or good/great