At the time Grantville was teleported into the past, Count Tilly was already infamous for allowing his forces to ravage and maraud the German countryside, forcing thousands of refugees to flee. The most notable example was the sacking of Magdeburg.
Tilly finally went too far when he invaded Saxony in September, 1631. Initially, the elector of Saxony, John George, vacillated rather than turn to Gustavus Adolphus of Sweden for help. However, once Tilly took Leipzig, John George finally formally combined his forces with Gustav's.
The Battle of Breitenfeld was on September 17, 1631. Tilly lost to Gustavus Adolphus and was forced to retreat. The defeat can in part be blamed on Gottfried Pappenheim, who, without orders, launched an attack that was repelled and crushed. However, it is also true that Tilly was simply out-marshalled by Gustavus. Tilly was injured repeatedly during the battle, but refused to surrender, allowing his men to retreat in the night.
Tilly's downfall came in April, 1632, at the Battle of Rain. Tilly had been camped across the Lech River. Gustavus deployed guns built in the New United States, and had the aid of sniper Julie Sims. Tilly's leg was smashed by a Swedish shell, and he was carried from the field. Gustavus forced the river. Gustavus offered to send his surgeon to Tilly. Tilly died two weeks later.