i don't think ranked matches give koins (other than the bonus ones through border/icon) so i've been grinding online tower battles. you get the normal amount + 1k if you're number 1.
The krypt itself has tons of koins to get inside. Once i finished story I went in and finished the whole crypt puzzles with just the koins from story. Also, I recommend doing klassic tower for every character before going into the krypt. You'll unlock their tournament costumes and in turn, where those locations were in the krypt become koin chests instead.
