How to increase spawn rates + some quetions

so ive een playing bromons for about a month now and I have almost all the common ones and even 2 rares and a few uncommon. i was wondering A what the formula for spanw rates is and how to increase spawn rates. and B how does the game decide what bromon to spawn? if I have all commons will a rarer one spawn? just now I had a mythical appear ( the cat one (sorry for not knowing the name) ) and I tried to fight it with my rare redditor and obviously lost but I was wondering if there’s like a way to wineven more unfair fights or wether its all luck. also what happens if I ignore the mythical? will a worse one spawn? thanks in advance!

Same, I have an epic and now I have seen rarely spawned Bromons. I only see commons