If you just need an instant understanding and not a deep one, fight Brawler like Mario, Gunner like Villager, and Swordfighter like Lucina. No one around here mains these three so I don't have deep insights, but just stay calm in the face of the unknown. Mii Brawler up-2 is like ZSS Boost Kick; watch out for that and otherwise he's like the third part of the Mario-Luigi dynamic kinda (similar sort of normals but somewhat worse normals than Mario in exchange for more mobility instead of generally better normals in exchange for worse mobility like Luigi). Any advice I try to give beyond that about Brawler (who is a legitimately good character) would be fraudulent so I hope that's enough. Mii Gunner has a lot of zoning tools but will struggle to kill you; be smart and toy around with what you can Gravitational Pull would be how I'd handle the threat of a Gunner player but you'll probably not run into one. Mii Swordfighter seems worse by the day; you can probably just improvise on the spot if someone picks him which is almost assuredly not going to happen.
