Yes, ribbit arena is decent. It takes a lot of time and it can be made a lot better. For example if you can position your fighters to form groups, because many times one fighter is fighting on his own and is getting an asswhooping. Very often it is the brawler who is getting destroyed.
Also the state of your fighters should be visible when buying upgrades.
I have never heard of brainrot. So it is something like twetch arcade? Is it still around?