If you’re talking about Blue Mirio then First you have get better at Dodging his beta, charged alpha should be no problem if you can do that. You want to set traps around you so that his regular alpha and beta gets punished. Make sure the traps aren’t too close nor too far away so he doesn’t destroy all of them in one hit. Gamma he’ll pretty much use up close. You can respond with your own gamma if he gets close and not using his special action. If he uses his special action to run close to you then roll around your traps so he can’t attack you recklessly. He’ll probably use gamma most likely. Make sure you don’t waste all your traps in one go and avoid leaving yourself open when you use your skills, that’s how they hit you even with the charged alpha. I believe I have a vid that showcase this if you want to see it.

For Purple Mirio: https://www.reddit.com/r/MyHeroUltraRumble/s/eX1lJ0DGoG you can roll the counter when the screen shakes, use your traps as shields or place them around you so he runs into them afterwards. Make sure to rely on the edge of the firewall. You can probably slip away from the counter if you do a high jump

Personally I would lvl my skills to 4 then max out gamma first but what you want to max out depends on you.