When playing as Richter and Maria once you gain a few levels and have enough MP you can use Richter's "Grand Cross" subweapon and deal massive damage (especially to enemies who are weak to 'Holy'-based attacks). The only problem is you can't move while performing it... with Richter.
Have Richter select the "Grand Cross" subweapon then swith over to Maria with "X" without having Richter out with her. Press "R" to have Richter come out and cast "Grand Cross". The moment he appears (he doesn't even have to have casted it yet) immediately press "A" to send Richter away and the "Grand Cross" will go to Maria except she can move around with it! This works well on bosses since you can pummel them with the cross but still be able to move out of harms way.