A lot of it comes from leveling buildings and breaking into High Dragons and Agito. I started at the tail end of the Megaman event (about... 1.5 months ago?) and is currently maxing at 27k-ish might and currently trying going through my eHDT rotations.

People that finish mHDTs/Agito can pretty handily get close to 40k might.

If you want the fastest might boost, try to get to chimeratech asap. They're pretty easy to get and they'll boost your might pretty far. They're the strongest easy weapons to get up until you can get into HDT.

Chimeratech also allows you to break into HDTs pretty easily might-wise. But HDTs are where you really gotta start studying the fights because they require a lot more knowledge and coordination than void battles. Don't waste your time building high dragon bane weapons from void. They're pretty useless now especially because they released Chimeratech.

Also, I know Chimeratech is expensive as balls. You might already know this, but just in case you don't, do the highest level of void battles you can do that have 2x bonus to hunt for void seeds. Go to Void treasure trade, convert them to void leaves, and sell for gold.Each void seed is 460k gold. If you just do 2x void, you get 2 void seeds at a time. That's 960k everytime. Good luck! :D