Musou Stars lacks a free & endless mode. You only have story mode, encyclopedia, gallery and options. If you want to replay missions you completed you'll have to redo that character/team's story mode again, no backtracking. New Game+ also only works during after the credits and can't be selected at the main menu.
The attributes or weapon system you'll have to use cards that provides bonuses or effects in battle, but yet they don't seem all that necessary to get the upper hand either. You can practically do Chaos mode with just being at max level.