My favoured strategy is to place my regular troops just north-east of Frankfurt (but still in the Hessen state), in 2 separate stacks, right on the border with Franken and Thuringen. When the civil war fires, I use the Landsturmregiments to make a frontline, while simply moving straight towards the CZE border with my two stacks of regulars, ensuring they take parallel (but 1 province apart) paths. You'll usually get intercepted by German forces, but since you have two different stacks, one stack will block the enemy troops, allowing your second stack to often reach the border. Then simply keep some regulars to secure the northern part (so they can't move in and re-link their lands), and use the rest to clean up the south. After you do, it's basically over. You may experience some intrusions on the northern side, but you should still be able to clean up the south before they manage to do too much damage.