So, anyone can correct me if I’m wrong. But mawile only spawns in bridge field, dusty bowl and giants mirror? Only found in blizzard weather, for dusty bowl and giants mirror it has around a 35% spawn rate (overworld). Meanwhile it has around a 10% spawn rate at bridge field (non-overworld).
There are also dens, such as den #82, and so on. But I play shield so I don’t directly know, but I looked into it so hopefully that helps! Minor thing if you didn’t know, (but I assume you do) is that mawile is a sword exclusive! So if your on shield you have to trade someone for it.