MP is gotten through either continually gaining experience with a lv 20 soul (50k exp for 1 MP I think?) or through exchanging soul points/hero soul points.

As far as Shingen goes, hero soul points are time gated. So a new player must wait X time before unlocking her. In the meantime, in theory, you can grind your ass off for an arbitrary number of MP via exp, as long as you still have the half-elixirs/seeds to keep you going.