Short answer is: gearing.
How you gear your champions determines how they will perform in battle, regardless what rarity they are. It's just math. Both Kael and Ronda can one-shot eachother if they are well geared, meaning geared for damage. It also means they have relatively poor defense stats. Ronda for instance has the advantage of putting a shield on herself, but this doesn't guarantee she will survive the hit. With all other things being equal, gear trumps champions.
If you post their gearing and stats, I am sure we can find a way to make them perform better.
Team power is only a rough indicator, and indeed I meet a lot of Kael teams that attempt to one-shot my offense in arena. I've therefore added a HP tank on shield runes to stop that happening - currently using Endalia, the new high elf from the fusion event. So far during this tournament I haven't lost to a single Kael team because of her. :)